package storage

import (
	"bytes"
	"container/heap"
	"errors"
	"fmt"
	"io"
	"path/filepath"
	"regexp"
	"sort"
	"strconv"
	"strings"
	"sync"
	"sync/atomic"
	"time"
	"unsafe"

	"github.com/VictoriaMetrics/metricsql"
	"github.com/cespare/xxhash/v2"

	"github.com/VictoriaMetrics/VictoriaMetrics/lib/atomicutil"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/bytesutil"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/encoding"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/fasttime"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/fs"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/lrucache"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/memory"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/mergeset"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/querytracer"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/slicesutil"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/uint64set"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/workingsetcache"
)

const (
	// Prefix for MetricName->TSID entries.
	//
	// This index is used only when -disablePerDayIndex flag is set.
	//
	// Otherwise, this index is substituted with nsPrefixDateMetricNameToTSID,
	// since the MetricName->TSID index may require big amounts of memory for
	// indexdb/dataBlocks cache when it grows big on the configured retention
	// under high churn rate (e.g. when new time series are constantly
	// registered).
	//
	// It is much more efficient from memory usage PoV to query per-day MetricName->TSID index
	// (aka nsPrefixDateMetricNameToTSID) when the TSID must be obtained for the given MetricName
	// during data ingestion under high churn rate and big retention.
	//
	nsPrefixMetricNameToTSID = 0

	// Prefix for Tag->MetricID entries.
	nsPrefixTagToMetricIDs = 1

	// Prefix for MetricID->TSID entries.
	nsPrefixMetricIDToTSID = 2

	// Prefix for MetricID->MetricName entries.
	nsPrefixMetricIDToMetricName = 3

	// Prefix for deleted MetricID entries.
	nsPrefixDeletedMetricID = 4

	// Prefix for Date->MetricID entries.
	nsPrefixDateToMetricID = 5

	// Prefix for (Date,Tag)->MetricID entries.
	nsPrefixDateTagToMetricIDs = 6

	// Prefix for (Date,MetricName)->TSID entries.
	nsPrefixDateMetricNameToTSID = 7
)

// indexDB represents an index db.
type indexDB struct {
	// The number of references to indexDB struct.
	refCount atomic.Int32

	// if the mustDrop is set to true, then the indexDB must be dropped after refCount reaches zero.
	mustDrop atomic.Bool

	// The number of calls for date range searches.
	dateRangeSearchCalls atomic.Uint64

	// The number of hits for date range searches.
	dateRangeSearchHits atomic.Uint64

	// The number of calls for global search.
	globalSearchCalls atomic.Uint64

	// The number of missing MetricID -> TSID entries.
	// High rate for this value means corrupted indexDB.
	missingTSIDsForMetricID atomic.Uint64

	// missingMetricNamesForMetricID is a counter of missing MetricID -> MetricName entries.
	// High rate may mean corrupted indexDB due to unclean shutdown.
	// The db must be automatically recovered after that.
	missingMetricNamesForMetricID atomic.Uint64

	// minMissingTimestampByKey holds the minimum timestamps by index search key,
	// which is missing in the given indexDB.
	// Key must be formed with marshalCommonPrefix function.
	//
	// This field is used at containsTimeRange() function only for the previous indexDB,
	// since this indexDB is readonly.
	// This field cannot be used for the current indexDB, since it may receive data
	// with bigger timestamps at any time.
	minMissingTimestampByKey map[string]int64
	// protects minMissingTimestampByKey
	minMissingTimestampByKeyLock sync.Mutex

	// generation identifies the index generation ID
	// and is used for syncing items from different indexDBs
	generation uint64

	name string
	tb   *mergeset.Table

	// noRegisterNewSeries indicates whether the indexDB receives new entries or
	// not.
	//
	// Note that setting this field to true won't disable registering new
	// index entries (should they arrive). It is solely used to decide whether
	// the containsTimeRange() optimization can be applied to the indexDB.
	noRegisterNewSeries atomic.Bool

	// Cache for fast TagFilters -> MetricIDs lookup.
	tagFiltersToMetricIDsCache *lrucache.Cache

	// The parent storage.
	s *Storage

	// Cache for (date, tagFilter) -> loopsCount, which is used for reducing
	// the amount of work when matching a set of filters.
	loopsPerDateTagFilterCache *workingsetcache.Cache

	// dateMetricIDCache is (date, metricID) cache that is used to speed up the
	// data ingestion by storing the is.hasDateMetricID() search results in
	// memory.
	dateMetricIDCache *dateMetricIDCache

	indexSearchPool sync.Pool
}

var maxTagFiltersCacheSize uint64

// SetTagFiltersCacheSize overrides the default size of tagFiltersToMetricIDsCache
func SetTagFiltersCacheSize(size int) {
	maxTagFiltersCacheSize = uint64(size)
}

func getTagFiltersCacheSize() uint64 {
	if maxTagFiltersCacheSize <= 0 {
		return uint64(float64(memory.Allowed()) / 32)
	}
	return maxTagFiltersCacheSize
}

// mustOpenIndexDB opens index db from the given path.
//
// The last segment of the path should contain unique hex value which
// will be then used as indexDB.generation
func mustOpenIndexDB(path string, s *Storage, isReadOnly *atomic.Bool, noRegisterNewSeries bool) *indexDB {
	if s == nil {
		logger.Panicf("BUG: Storage must be non-nil")
	}

	name := filepath.Base(path)
	gen, err := strconv.ParseUint(name, 16, 64)
	if err != nil {
		logger.Panicf("FATAL: cannot parse indexdb path %q: %s", path, err)
	}

	tfssCache := lrucache.NewCache(getTagFiltersCacheSize)
	tb := mergeset.MustOpenTable(path, dataFlushInterval, tfssCache.Reset, mergeTagToMetricIDsRows, isReadOnly)
	db := &indexDB{
		generation: gen,
		tb:         tb,
		name:       name,

		minMissingTimestampByKey:   make(map[string]int64),
		tagFiltersToMetricIDsCache: tfssCache,
		s:                          s,
		loopsPerDateTagFilterCache: workingsetcache.New(memory.Allowed() / 128),
		dateMetricIDCache:          newDateMetricIDCache(),
	}
	db.noRegisterNewSeries.Store(noRegisterNewSeries)
	db.incRef()
	return db
}

const noDeadline = 1<<64 - 1

// IndexDBMetrics contains essential metrics for indexDB.
type IndexDBMetrics struct {
	TagFiltersToMetricIDsCacheSize         uint64
	TagFiltersToMetricIDsCacheSizeBytes    uint64
	TagFiltersToMetricIDsCacheSizeMaxBytes uint64
	TagFiltersToMetricIDsCacheRequests     uint64
	TagFiltersToMetricIDsCacheMisses       uint64
	TagFiltersToMetricIDsCacheResets       uint64

	DateMetricIDCacheSize         uint64
	DateMetricIDCacheSizeBytes    uint64
	DateMetricIDCacheSizeMaxBytes uint64
	DateMetricIDCacheSyncsCount   uint64
	DateMetricIDCacheResetsCount  uint64

	IndexDBRefCount uint64

	RecentHourMetricIDsSearchCalls uint64
	RecentHourMetricIDsSearchHits  uint64

	DateRangeSearchCalls uint64
	DateRangeSearchHits  uint64
	GlobalSearchCalls    uint64

	MissingTSIDsForMetricID       uint64
	MissingMetricNamesForMetricID uint64

	IndexBlocksWithMetricIDsProcessed      uint64
	IndexBlocksWithMetricIDsIncorrectOrder uint64

	MinTimestampForCompositeIndex     uint64
	CompositeFilterSuccessConversions uint64
	CompositeFilterMissingConversions uint64

	mergeset.TableMetrics
}

func (db *indexDB) scheduleToDrop() {
	db.mustDrop.Store(true)
}

// UpdateMetrics updates m with metrics from the db.
func (db *indexDB) UpdateMetrics(m *IndexDBMetrics) {
	// global index metrics
	m.IndexBlocksWithMetricIDsProcessed = indexBlocksWithMetricIDsProcessed.Load()
	m.IndexBlocksWithMetricIDsIncorrectOrder = indexBlocksWithMetricIDsIncorrectOrder.Load()

	m.MinTimestampForCompositeIndex = uint64(db.s.minTimestampForCompositeIndex)
	m.CompositeFilterSuccessConversions = compositeFilterSuccessConversions.Load()
	m.CompositeFilterMissingConversions = compositeFilterMissingConversions.Load()

	m.TagFiltersToMetricIDsCacheSize += uint64(db.tagFiltersToMetricIDsCache.Len())
	m.TagFiltersToMetricIDsCacheSizeBytes += uint64(db.tagFiltersToMetricIDsCache.SizeBytes())
	m.TagFiltersToMetricIDsCacheSizeMaxBytes += uint64(db.tagFiltersToMetricIDsCache.SizeMaxBytes())
	m.TagFiltersToMetricIDsCacheRequests += db.tagFiltersToMetricIDsCache.Requests()
	m.TagFiltersToMetricIDsCacheMisses += db.tagFiltersToMetricIDsCache.Misses()
	m.TagFiltersToMetricIDsCacheResets += db.tagFiltersToMetricIDsCache.Resets()

	dmcs := db.dateMetricIDCache.Stats()
	m.DateMetricIDCacheSize += dmcs.Size
	m.DateMetricIDCacheSizeBytes += dmcs.SizeBytes
	m.DateMetricIDCacheSizeMaxBytes += dmcs.SizeMaxBytes
	m.DateMetricIDCacheSyncsCount += dmcs.SyncsCount
	m.DateMetricIDCacheResetsCount += dmcs.ResetsCount

	m.IndexDBRefCount += uint64(db.refCount.Load())

	m.DateRangeSearchCalls += db.dateRangeSearchCalls.Load()
	m.DateRangeSearchHits += db.dateRangeSearchHits.Load()
	m.GlobalSearchCalls += db.globalSearchCalls.Load()

	m.MissingTSIDsForMetricID += db.missingTSIDsForMetricID.Load()
	m.MissingMetricNamesForMetricID += db.missingMetricNamesForMetricID.Load()

	db.tb.UpdateMetrics(&m.TableMetrics)
}

// MustClose closes db.
func (db *indexDB) MustClose() {
	db.decRef()
}

func (db *indexDB) incRef() {
	db.refCount.Add(1)
}

func (db *indexDB) decRef() {
	n := db.refCount.Add(-1)
	if n < 0 {
		logger.Panicf("BUG: %q negative refCount: %d", db.name, n)
	}
	if n > 0 {
		return
	}

	tbPath := db.tb.Path()
	db.tb.MustClose()
	db.tb = nil

	// Free space occupied by caches owned by db.
	db.tagFiltersToMetricIDsCache.MustStop()
	db.loopsPerDateTagFilterCache.Stop()

	db.tagFiltersToMetricIDsCache = nil
	db.s = nil
	db.loopsPerDateTagFilterCache = nil

	if !db.mustDrop.Load() {
		return
	}

	logger.Infof("dropping indexDB %q", tbPath)
	fs.MustRemoveDir(tbPath)
	logger.Infof("indexDB %q has been dropped", tbPath)
}

// getMetricIDsFromTagFiltersCache retrieves the set of metricIDs that
// correspond to the given (tffs, tr) key.
//
// The caller must convert the (tfss, tr) to a byte slice and use it as the key
// when calling this method (see marshalTagFiltersKey()).
//
// The caller must not modify the set of metricIDs returned by this method.
func (db *indexDB) getMetricIDsFromTagFiltersCache(qt *querytracer.Tracer, key []byte) (*uint64set.Set, bool) {
	qt.Printf("search for metricIDs in tag filters cache")
	v := db.tagFiltersToMetricIDsCache.GetEntry(bytesutil.ToUnsafeString(key))
	if v == nil {
		qt.Printf("cache miss")
		return nil, false
	}
	metricIDs := v.(*uint64set.Set)
	qt.Printf("found %d metricIDs in cache", metricIDs.Len())
	return metricIDs, true
}

// putMetricIDsToTagFiltersCache stores the set of metricIDs that
// correspond to the given (tffs, tr) key into the cache.
//
// The caller must convert the (tfss, tr) to a byte slice and use it as the key
// when calling this method (see marshalTagFiltersKey()).
//
// The caller must not modify the set of metricIDs after calling this method.
func (db *indexDB) putMetricIDsToTagFiltersCache(qt *querytracer.Tracer, metricIDs *uint64set.Set, key []byte) {
	qt.Printf("put %d metricIDs in cache", metricIDs.Len())
	db.tagFiltersToMetricIDsCache.PutEntry(string(key), metricIDs)
	qt.Printf("stored %d metricIDs into cache", metricIDs.Len())
}

func (db *indexDB) getFromMetricIDCache(dst *TSID, metricID uint64) error {
	// There is no need in checking for deleted metricIDs here, since they
	// must be checked by the caller.
	buf := (*[unsafe.Sizeof(*dst)]byte)(unsafe.Pointer(dst))
	key := (*[unsafe.Sizeof(metricID)]byte)(unsafe.Pointer(&metricID))
	tmp := db.s.metricIDCache.Get(buf[:0], key[:])
	if len(tmp) == 0 {
		// The TSID for the given metricID wasn't found in the cache.
		return io.EOF
	}
	if &tmp[0] != &buf[0] || len(tmp) != len(buf) {
		return fmt.Errorf("corrupted MetricID->TSID cache: unexpected size for metricID=%d value; got %d bytes; want %d bytes", metricID, len(tmp), len(buf))
	}
	return nil
}

func (db *indexDB) putToMetricIDCache(metricID uint64, tsid *TSID) {
	buf := (*[unsafe.Sizeof(*tsid)]byte)(unsafe.Pointer(tsid))
	key := (*[unsafe.Sizeof(metricID)]byte)(unsafe.Pointer(&metricID))
	db.s.metricIDCache.Set(key[:], buf[:])
}

func marshalTagFiltersKey(dst []byte, tfss []*TagFilters, tr TimeRange) []byte {
	// Round start and end times to per-day granularity according to per-day inverted index.
	startDate, endDate := tr.DateRange()
	dst = encoding.MarshalUint64(dst, startDate)
	dst = encoding.MarshalUint64(dst, endDate)
	for _, tfs := range tfss {
		dst = append(dst, 0) // separator between tfs groups.
		for i := range tfs.tfs {
			dst = tfs.tfs[i].Marshal(dst)
		}
	}
	return dst
}

type indexSearch struct {
	db *indexDB
	ts mergeset.TableSearch
	kb bytesutil.ByteBuffer
	mp tagToMetricIDsRowParser

	// deadline in unix timestamp seconds for the given search.
	deadline uint64
}

// getIndexSearch returns an indexSearch with default configuration
func (db *indexDB) getIndexSearch(deadline uint64) *indexSearch {
	return db.getIndexSearchInternal(deadline, false)
}

func (db *indexDB) getIndexSearchInternal(deadline uint64, sparse bool) *indexSearch {
	v := db.indexSearchPool.Get()
	if v == nil {
		v = &indexSearch{
			db: db,
		}
	}
	is := v.(*indexSearch)
	is.ts.Init(db.tb, sparse)
	is.deadline = deadline
	return is
}

func (db *indexDB) putIndexSearch(is *indexSearch) {
	is.ts.MustClose()
	is.kb.Reset()
	is.mp.Reset()
	is.deadline = 0

	db.indexSearchPool.Put(is)
}

func generateTSID(dst *TSID, mn *MetricName) {
	dst.MetricGroupID = xxhash.Sum64(mn.MetricGroup)
	// Assume that the job-like metric is put at mn.Tags[0], while instance-like metric is put at mn.Tags[1]
	// This assumption is true because mn.Tags must be sorted with mn.sortTags() before calling generateTSID() function.
	// This allows grouping data blocks for the same (job, instance) close to each other on disk.
	// This reduces disk seeks and disk read IO when data blocks are read from disk for the same job and/or instance.
	// For example, data blocks for time series matching `process_resident_memory_bytes{job="vmstorage"}` are physically adjacent on disk.
	if len(mn.Tags) > 0 {
		dst.JobID = uint32(xxhash.Sum64(mn.Tags[0].Value))
	}
	if len(mn.Tags) > 1 {
		dst.InstanceID = uint32(xxhash.Sum64(mn.Tags[1].Value))
	}
	dst.MetricID = generateUniqueMetricID()
}

func (db *indexDB) createGlobalIndexes(tsid *TSID, mn *MetricName) {
	ii := getIndexItems()
	defer putIndexItems(ii)

	if db.s.disablePerDayIndex {
		// Create metricName -> TSID entry.
		// This index is used for searching a TSID by metric name during data
		// ingestion or metric name registration when -disablePerDayIndex flag
		// is set.
		ii.B = marshalCommonPrefix(ii.B, nsPrefixMetricNameToTSID)
		ii.B = mn.Marshal(ii.B)
		ii.B = append(ii.B, kvSeparatorChar)
		ii.B = tsid.Marshal(ii.B)
		ii.Next()
	}

	// Create metricID -> metricName entry.
	ii.B = marshalCommonPrefix(ii.B, nsPrefixMetricIDToMetricName)
	ii.B = encoding.MarshalUint64(ii.B, tsid.MetricID)
	ii.B = mn.Marshal(ii.B)
	ii.Next()

	// Create metricID -> TSID entry.
	ii.B = marshalCommonPrefix(ii.B, nsPrefixMetricIDToTSID)
	ii.B = encoding.MarshalUint64(ii.B, tsid.MetricID)
	ii.B = tsid.Marshal(ii.B)
	ii.Next()

	// Create tag -> metricID entries for every tag in mn.
	kb := kbPool.Get()
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs)
	ii.registerTagIndexes(kb.B, mn, tsid.MetricID)
	kbPool.Put(kb)

	db.tb.AddItems(ii.Items)
}

type indexItems struct {
	B     []byte
	Items [][]byte

	start int
}

func (ii *indexItems) reset() {
	ii.B = ii.B[:0]
	ii.Items = ii.Items[:0]
	ii.start = 0
}

func (ii *indexItems) Next() {
	ii.Items = append(ii.Items, ii.B[ii.start:])
	ii.start = len(ii.B)
}

func getIndexItems() *indexItems {
	v := indexItemsPool.Get()
	if v == nil {
		return &indexItems{}
	}
	return v.(*indexItems)
}

func putIndexItems(ii *indexItems) {
	ii.reset()
	indexItemsPool.Put(ii)
}

var indexItemsPool sync.Pool

// SearchLabelNames returns all the label names, which match the given tfss on
// the given tr.
func (db *indexDB) SearchLabelNames(qt *querytracer.Tracer, tfss []*TagFilters, tr TimeRange, maxLabelNames, maxMetrics int, deadline uint64) (map[string]struct{}, error) {
	qt = qt.NewChild("search for label names: filters=%s, timeRange=%s, maxLabelNames=%d, maxMetrics=%d", tfss, &tr, maxLabelNames, maxMetrics)
	defer qt.Done()

	is := db.getIndexSearch(deadline)
	lns, err := is.searchLabelNamesWithFiltersOnTimeRange(qt, tfss, tr, maxLabelNames, maxMetrics)
	db.putIndexSearch(is)
	if err != nil {
		return nil, err
	}

	qt.Printf("found %d label names", len(lns))
	return lns, nil
}

func (is *indexSearch) searchLabelNamesWithFiltersOnTimeRange(qt *querytracer.Tracer, tfss []*TagFilters, tr TimeRange, maxLabelNames, maxMetrics int) (map[string]struct{}, error) {
	if tr == globalIndexTimeRange {
		qtChild := qt.NewChild("search for label names in global index: filters=%s", tfss)
		lns, err := is.searchLabelNamesWithFiltersOnDate(qtChild, tfss, globalIndexDate, maxLabelNames, maxMetrics)
		qtChild.Done()
		return lns, err
	}

	minDate, maxDate := tr.DateRange()
	var mu sync.Mutex
	wg := getWaitGroup()
	var errGlobal error
	lns := make(map[string]struct{})
	qt = qt.NewChild("parallel search for label names: filters=%s, timeRange=%s", tfss, &tr)
	for date := minDate; date <= maxDate; date++ {
		wg.Add(1)
		qtChild := qt.NewChild("search for label names: filters=%s, date=%s", tfss, dateToString(date))
		go func(date uint64) {
			defer func() {
				qtChild.Done()
				wg.Done()
			}()
			isLocal := is.db.getIndexSearch(is.deadline)
			lnsLocal, err := isLocal.searchLabelNamesWithFiltersOnDate(qtChild, tfss, date, maxLabelNames, maxMetrics)
			is.db.putIndexSearch(isLocal)
			mu.Lock()
			defer mu.Unlock()
			if errGlobal != nil {
				return
			}
			if err != nil {
				errGlobal = err
				return
			}
			if len(lns) >= maxLabelNames {
				return
			}
			for k := range lnsLocal {
				lns[k] = struct{}{}
			}
		}(date)
	}
	wg.Wait()
	putWaitGroup(wg)
	qt.Done()
	return lns, errGlobal
}

func (is *indexSearch) searchLabelNamesWithFiltersOnDate(qt *querytracer.Tracer, tfss []*TagFilters, date uint64, maxLabelNames, maxMetrics int) (map[string]struct{}, error) {
	var filter *uint64set.Set
	if !isSingleMetricNameFilter(tfss) {
		filter, err := is.searchMetricIDsWithFiltersOnDate(qt, tfss, date, maxMetrics)
		if err != nil {
			return nil, err
		}
		if filter != nil && filter.Len() <= 100e3 {
			// It is faster to obtain label names by metricIDs from the filter
			// instead of scanning the inverted index for the matching filters.
			// This should help https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2978
			metricIDs := filter.AppendTo(nil)
			qt.Printf("sort %d metricIDs", len(metricIDs))
			lns := is.getLabelNamesForMetricIDs(qt, metricIDs, maxLabelNames)
			return lns, nil
		}
	}

	var prevLabelName []byte
	ts := &is.ts
	kb := &is.kb
	mp := &is.mp
	dmis := is.db.s.getDeletedMetricIDs()
	loopsPaceLimiter := 0
	underscoreNameSeen := false
	nsPrefixExpected := byte(nsPrefixDateTagToMetricIDs)
	if date == globalIndexDate {
		nsPrefixExpected = nsPrefixTagToMetricIDs
	}

	hasCompositeLabelName := false
	kb.B = is.marshalCommonPrefixForDate(kb.B[:0], date)
	if name := getCommonMetricNameForTagFilterss(tfss); len(name) > 0 {
		compositeLabelName := marshalCompositeTagKey(nil, name, nil)
		kb.B = marshalTagValue(kb.B, compositeLabelName)
		// Drop trailing tagSeparator
		kb.B = kb.B[:len(kb.B)-1]
		hasCompositeLabelName = true
	}
	prefix := append([]byte{}, kb.B...)

	ts.Seek(prefix)
	lns := make(map[string]struct{})
	for len(lns) < maxLabelNames && ts.NextItem() {
		if loopsPaceLimiter&paceLimiterFastIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return nil, err
			}
		}
		loopsPaceLimiter++
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			break
		}
		if err := mp.Init(item, nsPrefixExpected); err != nil {
			return nil, err
		}
		if mp.GetMatchingSeriesCount(filter, dmis) == 0 {
			continue
		}
		labelName := mp.Tag.Key
		if len(labelName) == 0 || hasCompositeLabelName {
			underscoreNameSeen = true
		}
		if (!hasCompositeLabelName && isArtificialTagKey(labelName)) || string(labelName) == string(prevLabelName) {
			// Search for the next tag key.
			// The last char in kb.B must be tagSeparatorChar.
			// Just increment it in order to jump to the next tag key.
			kb.B = is.marshalCommonPrefixForDate(kb.B[:0], date)
			if !hasCompositeLabelName && len(labelName) > 0 && labelName[0] == compositeTagKeyPrefix {
				// skip composite tag entries
				kb.B = append(kb.B, compositeTagKeyPrefix)
			} else {
				kb.B = marshalTagValue(kb.B, labelName)
			}
			kb.B[len(kb.B)-1]++
			ts.Seek(kb.B)
			continue
		}
		if !hasCompositeLabelName {
			lns[string(labelName)] = struct{}{}
		} else {
			_, key, err := unmarshalCompositeTagKey(labelName)
			if err != nil {
				return nil, fmt.Errorf("cannot unmarshal composite tag key: %s", err)
			}
			lns[string(key)] = struct{}{}
		}
		prevLabelName = append(prevLabelName[:0], labelName...)
	}
	if underscoreNameSeen {
		lns["__name__"] = struct{}{}
	}
	if err := ts.Error(); err != nil {
		return nil, fmt.Errorf("error during search for prefix %q: %w", prefix, err)
	}
	return lns, nil
}

func (is *indexSearch) getLabelNamesForMetricIDs(qt *querytracer.Tracer, metricIDs []uint64, maxLabelNames int) map[string]struct{} {
	lns := make(map[string]struct{})
	if len(metricIDs) > 0 {
		lns["__name__"] = struct{}{}
	}

	dmis := is.db.s.getDeletedMetricIDs()

	var mn MetricName
	foundLabelNames := 0
	var buf []byte
	for _, metricID := range metricIDs {
		if dmis.Has(metricID) {
			// skip deleted IDs from result
			continue
		}
		var ok bool
		buf, ok = is.searchMetricNameWithCache(buf[:0], metricID)
		if !ok {
			// It is likely the metricID->metricName entry didn't propagate to inverted index yet.
			// Skip this metricID for now.
			continue
		}
		if err := mn.Unmarshal(buf); err != nil {
			logger.Panicf("FATAL: cannot unmarshal metricName %q: %s", buf, err)
		}
		for _, tag := range mn.Tags {
			if _, ok := lns[string(tag.Key)]; !ok {
				foundLabelNames++
				lns[string(tag.Key)] = struct{}{}
				if len(lns) >= maxLabelNames {
					qt.Printf("hit the limit on the number of unique label names: %d", maxLabelNames)
					return lns
				}
			}
		}
	}
	qt.Printf("get %d distinct label names from %d metricIDs", foundLabelNames, len(metricIDs))
	return lns
}

// SearchLabelValues returns label values for the given labelName, tfss and tr.
func (db *indexDB) SearchLabelValues(qt *querytracer.Tracer, labelName string, tfss []*TagFilters, tr TimeRange, maxLabelValues, maxMetrics int, deadline uint64) (map[string]struct{}, error) {
	qt = qt.NewChild("search for label values: labelName=%q, filters=%s, timeRange=%s, maxLabelNames=%d, maxMetrics=%d", labelName, tfss, &tr, maxLabelValues, maxMetrics)
	defer qt.Done()

	key := labelName
	if key == "__name__" {
		key = ""
	}
	if len(tfss) == 1 && len(tfss[0].tfs) == 1 && string(tfss[0].tfs[0].key) == key {
		// tfss contains only a single filter on labelName. It is faster searching for label values
		// without any filters and limits and then later applying the filter and the limit to the found label values.
		qt.Printf("search for up to %d values for the label %q on the time range %s", maxMetrics, labelName, &tr)

		is := db.getIndexSearch(deadline)
		lvs, err := is.searchLabelValuesOnTimeRange(qt, labelName, nil, tr, maxMetrics, maxMetrics)
		db.putIndexSearch(is)
		if err != nil {
			return nil, err
		}

		needSlowSearch := len(lvs) == maxMetrics

		lvsLen := len(lvs)
		filterLabelValues(lvs, &tfss[0].tfs[0], key)
		qt.Printf("found %d out of %d values for the label %q after filtering", len(lvs), lvsLen, labelName)
		if len(lvs) >= maxLabelValues {
			qt.Printf("leave %d out of %d values for the label %q because of the limit", maxLabelValues, len(lvs), labelName)

			// We found at least maxLabelValues unique values for the label with the given filters.
			// It is OK returning all these values instead of falling back to the slow search.
			needSlowSearch = false
		}
		if !needSlowSearch {
			qt.Printf("found %d label values", len(lvs))
			return lvs, nil
		}
		qt.Printf("fall back to slow search because only a subset of label values is found")
	}

	is := db.getIndexSearch(deadline)
	lvs, err := is.searchLabelValuesOnTimeRange(qt, labelName, tfss, tr, maxMetrics, maxMetrics)
	db.putIndexSearch(is)
	if err != nil {
		return nil, err
	}

	qt.Printf("found %d label values", len(lvs))
	return lvs, nil
}

func filterLabelValues(lvs map[string]struct{}, tf *tagFilter, key string) {
	var b []byte
	for lv := range lvs {
		b = marshalCommonPrefix(b[:0], nsPrefixTagToMetricIDs)
		b = marshalTagValue(b, bytesutil.ToUnsafeBytes(key))
		b = marshalTagValue(b, bytesutil.ToUnsafeBytes(lv))
		ok, err := tf.match(b)
		if err != nil {
			logger.Panicf("BUG: cannot match label %q=%q with tagFilter %s: %w", key, lv, tf.String(), err)
		}
		if !ok {
			delete(lvs, lv)
		}
	}
}

func (is *indexSearch) searchLabelValuesOnTimeRange(qt *querytracer.Tracer, labelName string, tfss []*TagFilters, tr TimeRange, maxLabelValues, maxMetrics int) (map[string]struct{}, error) {
	if tr == globalIndexTimeRange {
		qtChild := qt.NewChild("search for label values in global index: labelName=%q, filters=%s", labelName, tfss)
		lvs, err := is.searchLabelValuesOnDate(qtChild, labelName, tfss, globalIndexDate, maxLabelValues, maxMetrics)
		qtChild.Done()

		// Skip empty values, since they have no any meaning.
		// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/600
		delete(lvs, "")

		return lvs, err
	}

	minDate, maxDate := tr.DateRange()
	var mu sync.Mutex
	wg := getWaitGroup()
	var errGlobal error
	lvs := make(map[string]struct{})
	qt = qt.NewChild("parallel search for label values: labelName=%q, filters=%s, timeRange=%s", labelName, tfss, &tr)
	for date := minDate; date <= maxDate; date++ {
		wg.Add(1)
		qtChild := qt.NewChild("search for label values: filters=%s, date=%s", tfss, dateToString(date))
		go func(date uint64) {
			defer func() {
				qtChild.Done()
				wg.Done()
			}()
			isLocal := is.db.getIndexSearch(is.deadline)
			lvsLocal, err := isLocal.searchLabelValuesOnDate(qtChild, labelName, tfss, date, maxLabelValues, maxMetrics)
			is.db.putIndexSearch(isLocal)
			mu.Lock()
			defer mu.Unlock()
			if errGlobal != nil {
				return
			}
			if err != nil {
				errGlobal = err
				return
			}
			if len(lvs) >= maxLabelValues {
				return
			}
			for v := range lvsLocal {
				lvs[v] = struct{}{}
			}
		}(date)
	}
	wg.Wait()
	putWaitGroup(wg)
	qt.Done()

	// Skip empty values, since they have no any meaning.
	// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/600
	delete(lvs, "")

	return lvs, errGlobal
}

func (is *indexSearch) searchLabelValuesOnDate(qt *querytracer.Tracer, labelName string, tfss []*TagFilters, date uint64, maxLabelValues, maxMetrics int) (map[string]struct{}, error) {
	if labelName == "__name__" {
		// __name__ label is encoded as empty string in indexdb.
		labelName = ""
	}
	useCompositeScan := labelName != "" && isSingleMetricNameFilter(tfss)
	var filter *uint64set.Set
	if !useCompositeScan {
		filter, err := is.searchMetricIDsWithFiltersOnDate(qt, tfss, date, maxMetrics)
		if err != nil {
			return nil, err
		}
		if filter != nil && filter.Len() <= 100e3 {
			// It is faster to obtain label values by metricIDs from the filter
			// instead of scanning the inverted index for the matching filters.
			// This should help https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2978
			metricIDs := filter.AppendTo(nil)
			qt.Printf("sort %d metricIDs", len(metricIDs))
			lvs := is.getLabelValuesForMetricIDs(qt, labelName, metricIDs, maxLabelValues)
			return lvs, nil
		}
	}

	labelNameBytes := bytesutil.ToUnsafeBytes(labelName)
	if name := getCommonMetricNameForTagFilterss(tfss); len(name) > 0 && labelName != "" {
		labelNameBytes = marshalCompositeTagKey(nil, name, labelNameBytes)
	}

	lvs := make(map[string]struct{})
	var prevLabelValue []byte
	ts := &is.ts
	kb := &is.kb
	mp := &is.mp
	dmis := is.db.s.getDeletedMetricIDs()
	loopsPaceLimiter := 0
	nsPrefixExpected := byte(nsPrefixDateTagToMetricIDs)
	if date == globalIndexDate {
		nsPrefixExpected = nsPrefixTagToMetricIDs
	}
	kb.B = is.marshalCommonPrefixForDate(kb.B[:0], date)
	kb.B = marshalTagValue(kb.B, labelNameBytes)
	prefix := append([]byte{}, kb.B...)
	ts.Seek(prefix)
	for len(lvs) < maxLabelValues && ts.NextItem() {
		if loopsPaceLimiter&paceLimiterFastIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return nil, err
			}
		}
		loopsPaceLimiter++
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			break
		}
		if err := mp.Init(item, nsPrefixExpected); err != nil {
			return nil, err
		}
		if mp.GetMatchingSeriesCount(filter, dmis) == 0 {
			continue
		}
		labelValue := mp.Tag.Value
		if string(labelValue) == string(prevLabelValue) {
			// Search for the next tag value.
			// The last char in kb.B must be tagSeparatorChar.
			// Just increment it in order to jump to the next tag value.
			kb.B = is.marshalCommonPrefixForDate(kb.B[:0], date)
			kb.B = marshalTagValue(kb.B, labelNameBytes)
			kb.B = marshalTagValue(kb.B, labelValue)
			kb.B[len(kb.B)-1]++
			ts.Seek(kb.B)
			continue
		}
		lvs[string(labelValue)] = struct{}{}
		prevLabelValue = append(prevLabelValue[:0], labelValue...)
	}
	if err := ts.Error(); err != nil {
		return nil, fmt.Errorf("error when searching for tag name prefix %q: %w", prefix, err)
	}
	return lvs, nil
}

func (is *indexSearch) getLabelValuesForMetricIDs(qt *querytracer.Tracer, labelName string, metricIDs []uint64, maxLabelValues int) map[string]struct{} {
	if labelName == "" {
		labelName = "__name__"
	}

	lvs := make(map[string]struct{})
	dmis := is.db.s.getDeletedMetricIDs()

	var mn MetricName
	foundLabelValues := 0
	var buf []byte
	for _, metricID := range metricIDs {
		if dmis.Has(metricID) {
			// skip deleted IDs from result
			continue
		}
		var ok bool
		buf, ok = is.searchMetricNameWithCache(buf[:0], metricID)
		if !ok {
			// It is likely the metricID->metricName entry didn't propagate to inverted index yet.
			// Skip this metricID for now.
			continue
		}
		if err := mn.Unmarshal(buf); err != nil {
			logger.Panicf("FATAL: cannot unmarshal metricName %q: %s", buf, err)
		}
		tagValue := mn.GetTagValue(labelName)
		if _, ok := lvs[string(tagValue)]; !ok {
			foundLabelValues++
			lvs[string(tagValue)] = struct{}{}
			if len(lvs) >= maxLabelValues {
				qt.Printf("hit the limit on the number of unique label values for label %q: %d", labelName, maxLabelValues)
				return lvs
			}
		}
	}
	qt.Printf("get %d distinct values for label %q from %d metricIDs", foundLabelValues, labelName, len(metricIDs))
	return lvs
}

// SearchTagValueSuffixes returns all the tag value suffixes for the given tagKey and tagValuePrefix on the given tr.
//
// This allows implementing https://graphite-api.readthedocs.io/en/latest/api.html#metrics-find or similar APIs.
//
// If it returns maxTagValueSuffixes suffixes, then it is likely more than maxTagValueSuffixes suffixes is found.
func (db *indexDB) SearchTagValueSuffixes(qt *querytracer.Tracer, tr TimeRange, tagKey, tagValuePrefix string, delimiter byte, maxTagValueSuffixes int, deadline uint64) (map[string]struct{}, error) {
	qt = qt.NewChild("search tag value suffixes for timeRange=%s, tagKey=%q, tagValuePrefix=%q, delimiter=%c, maxTagValueSuffixes=%d",
		&tr, tagKey, tagValuePrefix, delimiter, maxTagValueSuffixes)
	defer qt.Done()

	// TODO: cache results?

	is := db.getIndexSearch(deadline)
	tvss, err := is.searchTagValueSuffixesForTimeRange(tr, tagKey, tagValuePrefix, delimiter, maxTagValueSuffixes)
	db.putIndexSearch(is)
	if err != nil {
		return nil, err
	}

	// Do not skip empty suffixes, since they may represent leaf tag values.

	qt.Printf("found %d suffixes", len(tvss))
	return tvss, nil
}

func (is *indexSearch) searchTagValueSuffixesForTimeRange(tr TimeRange, tagKey, tagValuePrefix string, delimiter byte, maxTagValueSuffixes int) (map[string]struct{}, error) {
	if tr == globalIndexTimeRange {
		return is.searchTagValueSuffixesAll(tagKey, tagValuePrefix, delimiter, maxTagValueSuffixes)
	}

	minDate, maxDate := tr.DateRange()
	// Query over multiple days in parallel.
	wg := getWaitGroup()
	var errGlobal error
	var mu sync.Mutex // protects tvss + errGlobal from concurrent access below.
	tvss := make(map[string]struct{})
	for minDate <= maxDate {
		wg.Add(1)
		go func(date uint64) {
			defer wg.Done()
			isLocal := is.db.getIndexSearch(is.deadline)
			tvssLocal, err := isLocal.searchTagValueSuffixesForDate(date, tagKey, tagValuePrefix, delimiter, maxTagValueSuffixes)
			is.db.putIndexSearch(isLocal)
			mu.Lock()
			defer mu.Unlock()
			if errGlobal != nil {
				return
			}
			if err != nil {
				errGlobal = err
				return
			}
			if len(tvss) > maxTagValueSuffixes {
				return
			}
			for k := range tvssLocal {
				tvss[k] = struct{}{}
			}
		}(minDate)
		minDate++
	}
	wg.Wait()
	putWaitGroup(wg)
	return tvss, errGlobal
}

func (is *indexSearch) searchTagValueSuffixesAll(tagKey, tagValuePrefix string, delimiter byte, maxTagValueSuffixes int) (map[string]struct{}, error) {
	kb := &is.kb
	nsPrefix := byte(nsPrefixTagToMetricIDs)
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefix)
	kb.B = marshalTagValue(kb.B, bytesutil.ToUnsafeBytes(tagKey))
	kb.B = marshalTagValue(kb.B, bytesutil.ToUnsafeBytes(tagValuePrefix))
	kb.B = kb.B[:len(kb.B)-1] // remove tagSeparatorChar from the end of kb.B
	prefix := append([]byte(nil), kb.B...)
	return is.searchTagValueSuffixesForPrefix(nsPrefix, prefix, len(tagValuePrefix), delimiter, maxTagValueSuffixes)
}

func (is *indexSearch) searchTagValueSuffixesForDate(date uint64, tagKey, tagValuePrefix string, delimiter byte, maxTagValueSuffixes int) (map[string]struct{}, error) {
	nsPrefix := byte(nsPrefixDateTagToMetricIDs)
	kb := &is.kb
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefix)
	kb.B = encoding.MarshalUint64(kb.B, date)
	kb.B = marshalTagValue(kb.B, bytesutil.ToUnsafeBytes(tagKey))
	kb.B = marshalTagValue(kb.B, bytesutil.ToUnsafeBytes(tagValuePrefix))
	kb.B = kb.B[:len(kb.B)-1] // remove tagSeparatorChar from the end of kb.B
	prefix := append([]byte(nil), kb.B...)
	return is.searchTagValueSuffixesForPrefix(nsPrefix, prefix, len(tagValuePrefix), delimiter, maxTagValueSuffixes)
}

func (is *indexSearch) searchTagValueSuffixesForPrefix(nsPrefix byte, prefix []byte, tagValuePrefixLen int, delimiter byte, maxTagValueSuffixes int) (map[string]struct{}, error) {
	kb := &is.kb
	ts := &is.ts
	mp := &is.mp
	dmis := is.db.s.getDeletedMetricIDs()
	loopsPaceLimiter := 0
	ts.Seek(prefix)
	tvss := make(map[string]struct{})
	for len(tvss) < maxTagValueSuffixes && ts.NextItem() {
		if loopsPaceLimiter&paceLimiterFastIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return nil, err
			}
		}
		loopsPaceLimiter++
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			break
		}
		if err := mp.Init(item, nsPrefix); err != nil {
			return nil, err
		}
		if mp.GetMatchingSeriesCount(nil, dmis) == 0 {
			continue
		}
		tagValue := mp.Tag.Value
		suffix := tagValue[tagValuePrefixLen:]
		n := bytes.IndexByte(suffix, delimiter)
		if n < 0 {
			// Found leaf tag value that doesn't have delimiters after the given tagValuePrefix.
			tvss[string(suffix)] = struct{}{}
			continue
		}
		// Found non-leaf tag value. Extract suffix that end with the given delimiter.
		suffix = suffix[:n+1]
		tvss[string(suffix)] = struct{}{}
		if suffix[len(suffix)-1] == 255 {
			continue
		}
		// Search for the next suffix
		suffix[len(suffix)-1]++
		kb.B = append(kb.B[:0], prefix...)
		kb.B = marshalTagValue(kb.B, suffix)
		kb.B = kb.B[:len(kb.B)-1] // remove tagSeparatorChar
		ts.Seek(kb.B)
	}
	if err := ts.Error(); err != nil {
		return nil, fmt.Errorf("error when searching for tag value suffixes for prefix %q: %w", prefix, err)
	}
	return tvss, nil
}

func (db *indexDB) SearchGraphitePaths(qt *querytracer.Tracer, tr TimeRange, qHead, qTail []byte, maxPaths int, deadline uint64) (map[string]struct{}, error) {
	qt = qt.NewChild("search for graphite paths: timeRange=%s, qHead=%q, qTail=%q, maxPaths=%d", &tr, bytesutil.ToUnsafeString(qHead), bytesutil.ToUnsafeString(qTail), maxPaths)
	defer qt.Done()

	n := bytes.IndexAny(qTail, "*[{")
	if n < 0 {
		// Verify that qHead matches a metric name.
		qHead = append(qHead, qTail...)
		suffixes, err := db.SearchTagValueSuffixes(qt, tr, "", bytesutil.ToUnsafeString(qHead), '.', 1, deadline)
		if err != nil {
			return nil, err
		}
		if len(suffixes) == 0 {
			// The query doesn't match anything.
			return nil, nil
		}
		// The map should contain just one element. The code below is an attempt
		// to implement suffixes[0] if it were a slice.
		for s := range suffixes {
			if len(s) > 0 {
				// The query matches a metric name with additional suffix.
				return nil, nil
			}
			break
		}
		return map[string]struct{}{string(qHead): {}}, nil
	}
	qHead = append(qHead, qTail[:n]...)
	suffixes, err := db.SearchTagValueSuffixes(qt, tr, "", bytesutil.ToUnsafeString(qHead), '.', maxPaths, deadline)
	if err != nil {
		return nil, err
	}
	if len(suffixes) == 0 {
		return nil, nil
	}
	if len(suffixes) >= maxPaths {
		return nil, fmt.Errorf("more than maxPaths=%d suffixes found", maxPaths)
	}
	qNode := qTail[n:]
	qTail = nil
	mustMatchLeafs := true
	if m := bytes.IndexByte(qNode, '.'); m >= 0 {
		qTail = qNode[m+1:]
		qNode = qNode[:m+1]
		mustMatchLeafs = false
	}
	re, err := getRegexpForGraphiteQuery(string(qNode))
	if err != nil {
		return nil, err
	}
	qHeadLen := len(qHead)
	paths := make(map[string]struct{})
	for suffix := range suffixes {
		if len(paths) > maxPaths {
			return nil, fmt.Errorf("more than maxPath=%d paths found", maxPaths)
		}
		if !re.MatchString(suffix) {
			continue
		}
		if mustMatchLeafs {
			qHead = append(qHead[:qHeadLen], suffix...)
			paths[string(qHead)] = struct{}{}
			continue
		}
		qHead = append(qHead[:qHeadLen], suffix...)
		ps, err := db.SearchGraphitePaths(qt, tr, qHead, qTail, maxPaths, deadline)
		if err != nil {
			return nil, err
		}
		for p := range ps {
			paths[p] = struct{}{}
		}
	}
	return paths, nil
}

func getRegexpForGraphiteQuery(q string) (*regexp.Regexp, error) {
	parts, tail := getRegexpPartsForGraphiteQuery(q)
	if len(tail) > 0 {
		return nil, fmt.Errorf("unexpected tail left after parsing %q: %q", q, tail)
	}
	reStr := "^" + strings.Join(parts, "") + "$"
	return metricsql.CompileRegexp(reStr)
}

func getRegexpPartsForGraphiteQuery(q string) ([]string, string) {
	var parts []string
	for {
		n := strings.IndexAny(q, "*{}[,")
		if n < 0 {
			parts = append(parts, regexp.QuoteMeta(q))
			return parts, ""
		}
		parts = append(parts, regexp.QuoteMeta(q[:n]))
		q = q[n:]
		switch q[0] {
		case ',', '}':
			return parts, q
		case '*':
			parts = append(parts, "[^.]*")
			q = q[1:]
		case '{':
			var tmp []string
			for {
				a, tail := getRegexpPartsForGraphiteQuery(q[1:])
				tmp = append(tmp, strings.Join(a, ""))
				if len(tail) == 0 {
					parts = append(parts, regexp.QuoteMeta("{"))
					parts = append(parts, strings.Join(tmp, ","))
					return parts, ""
				}
				if tail[0] == ',' {
					q = tail
					continue
				}
				if tail[0] == '}' {
					if len(tmp) == 1 {
						parts = append(parts, tmp[0])
					} else {
						parts = append(parts, "(?:"+strings.Join(tmp, "|")+")")
					}
					q = tail[1:]
					break
				}
				logger.Panicf("BUG: unexpected first char at tail %q; want `.` or `}`", tail)
			}
		case '[':
			n := strings.IndexByte(q, ']')
			if n < 0 {
				parts = append(parts, regexp.QuoteMeta(q))
				return parts, ""
			}
			parts = append(parts, q[:n+1])
			q = q[n+1:]
		}
	}
}

// GetSeriesCount returns the approximate number of unique timeseries in the db.
//
// It includes the deleted series.
func (db *indexDB) GetSeriesCount(deadline uint64) (uint64, error) {
	is := db.getIndexSearch(deadline)
	defer db.putIndexSearch(is)
	return is.getSeriesCount()
}

func (is *indexSearch) getSeriesCount() (uint64, error) {
	ts := &is.ts
	kb := &is.kb
	mp := &is.mp
	loopsPaceLimiter := 0
	var metricIDsLen uint64
	// Extract the number of series from ((__name__=value): metricIDs) rows
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs)
	kb.B = marshalTagValue(kb.B, nil)
	ts.Seek(kb.B)
	for ts.NextItem() {
		if loopsPaceLimiter&paceLimiterFastIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return 0, err
			}
		}
		loopsPaceLimiter++
		item := ts.Item
		if !bytes.HasPrefix(item, kb.B) {
			break
		}
		tail := item[len(kb.B):]
		n := bytes.IndexByte(tail, tagSeparatorChar)
		if n < 0 {
			return 0, fmt.Errorf("invalid tag->metricIDs line %q: cannot find tagSeparatorChar %d", item, tagSeparatorChar)
		}
		tail = tail[n+1:]
		if err := mp.InitOnlyTail(item, tail); err != nil {
			return 0, err
		}
		// Take into account deleted timeseries too.
		// It is OK if series can be counted multiple times in rare cases -
		// the returned number is an estimation.
		metricIDsLen += uint64(mp.MetricIDsLen())
	}
	if err := ts.Error(); err != nil {
		return 0, fmt.Errorf("error when counting unique timeseries: %w", err)
	}
	return metricIDsLen, nil
}

// GetTSDBStatus returns topN entries for tsdb status for the given tfss, date and focusLabel.
func (db *indexDB) GetTSDBStatus(qt *querytracer.Tracer, tfss []*TagFilters, date uint64, focusLabel string, topN, maxMetrics int, deadline uint64) (*TSDBStatus, error) {
	qt = qt.NewChild("collect TSDB status: filters=%s, date=%d, focusLabel=%q, topN=%d, maxMetrics=%d", tfss, date, focusLabel, topN, maxMetrics)
	defer qt.Done()

	is := db.getIndexSearch(deadline)
	defer db.putIndexSearch(is)
	return is.getTSDBStatus(qt, tfss, date, focusLabel, topN, maxMetrics)
}

// getTSDBStatus returns topN entries for tsdb status for the given tfss, date and focusLabel.
func (is *indexSearch) getTSDBStatus(qt *querytracer.Tracer, tfss []*TagFilters, date uint64, focusLabel string, topN, maxMetrics int) (*TSDBStatus, error) {
	filter, err := is.searchMetricIDsWithFiltersOnDate(qt, tfss, date, maxMetrics)
	if err != nil {
		return nil, err
	}
	if filter != nil && filter.Len() == 0 {
		qt.Printf("no matching series for filter=%s", tfss)
		return &TSDBStatus{}, nil
	}

	ts := &is.ts
	kb := &is.kb
	mp := &is.mp
	dmis := is.db.s.getDeletedMetricIDs()
	thSeriesCountByMetricName := newTopHeap(topN)
	thSeriesCountByLabelName := newTopHeap(topN)
	thSeriesCountByFocusLabelValue := newTopHeap(topN)
	thSeriesCountByLabelValuePair := newTopHeap(topN)
	thLabelValueCountByLabelName := newTopHeap(topN)
	var tmp, prevLabelName, prevLabelValuePair []byte
	var labelValueCountByLabelName, seriesCountByLabelValuePair uint64
	var totalSeries, labelSeries, totalLabelValuePairs uint64
	nameEqualBytes := []byte("__name__=")
	focusLabelEqualBytes := []byte(focusLabel + "=")

	loopsPaceLimiter := 0
	nsPrefixExpected := byte(nsPrefixDateTagToMetricIDs)
	if date == globalIndexDate {
		nsPrefixExpected = nsPrefixTagToMetricIDs
	}
	kb.B = is.marshalCommonPrefixForDate(kb.B[:0], date)
	prefix := append([]byte{}, kb.B...)
	ts.Seek(prefix)
	for ts.NextItem() {
		if loopsPaceLimiter&paceLimiterFastIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return nil, err
			}
		}
		loopsPaceLimiter++
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			break
		}
		if err := mp.Init(item, nsPrefixExpected); err != nil {
			return nil, err
		}
		matchingSeriesCount := mp.GetMatchingSeriesCount(filter, dmis)
		if matchingSeriesCount == 0 {
			// Skip rows without matching metricIDs.
			continue
		}
		tmp = append(tmp[:0], mp.Tag.Key...)
		labelName := tmp
		if isArtificialTagKey(labelName) {
			// Skip artificially created tag keys.
			kb.B = append(kb.B[:0], prefix...)
			if len(labelName) > 0 && labelName[0] == compositeTagKeyPrefix {
				kb.B = append(kb.B, compositeTagKeyPrefix)
			} else {
				kb.B = marshalTagValue(kb.B, labelName)
			}
			kb.B[len(kb.B)-1]++
			ts.Seek(kb.B)
			continue
		}
		if len(labelName) == 0 {
			labelName = append(labelName, "__name__"...)
			tmp = labelName
		}
		if string(labelName) == "__name__" {
			totalSeries += uint64(matchingSeriesCount)
		}
		tmp = append(tmp, '=')
		tmp = append(tmp, mp.Tag.Value...)
		labelValuePair := tmp
		if len(prevLabelName) == 0 {
			prevLabelName = append(prevLabelName[:0], labelName...)
		}
		if string(labelName) != string(prevLabelName) {
			thLabelValueCountByLabelName.push(prevLabelName, labelValueCountByLabelName)
			thSeriesCountByLabelName.push(prevLabelName, labelSeries)
			labelSeries = 0
			labelValueCountByLabelName = 0
			prevLabelName = append(prevLabelName[:0], labelName...)
		}
		if len(prevLabelValuePair) == 0 {
			prevLabelValuePair = append(prevLabelValuePair[:0], labelValuePair...)
			labelValueCountByLabelName++
		}
		if string(labelValuePair) != string(prevLabelValuePair) {
			thSeriesCountByLabelValuePair.push(prevLabelValuePair, seriesCountByLabelValuePair)
			if bytes.HasPrefix(prevLabelValuePair, nameEqualBytes) {
				thSeriesCountByMetricName.push(prevLabelValuePair[len(nameEqualBytes):], seriesCountByLabelValuePair)
			}
			if bytes.HasPrefix(prevLabelValuePair, focusLabelEqualBytes) {
				thSeriesCountByFocusLabelValue.push(prevLabelValuePair[len(focusLabelEqualBytes):], seriesCountByLabelValuePair)
			}
			seriesCountByLabelValuePair = 0
			labelValueCountByLabelName++
			prevLabelValuePair = append(prevLabelValuePair[:0], labelValuePair...)
		}
		// It is OK if series can be counted multiple times in rare cases -
		// the returned number is an estimation.
		labelSeries += uint64(matchingSeriesCount)
		seriesCountByLabelValuePair += uint64(matchingSeriesCount)
		totalLabelValuePairs += uint64(matchingSeriesCount)
	}
	if err := ts.Error(); err != nil {
		return nil, fmt.Errorf("error when counting time series by metric names: %w", err)
	}
	thLabelValueCountByLabelName.push(prevLabelName, labelValueCountByLabelName)
	thSeriesCountByLabelName.push(prevLabelName, labelSeries)
	thSeriesCountByLabelValuePair.push(prevLabelValuePair, seriesCountByLabelValuePair)
	if bytes.HasPrefix(prevLabelValuePair, nameEqualBytes) {
		thSeriesCountByMetricName.push(prevLabelValuePair[len(nameEqualBytes):], seriesCountByLabelValuePair)
	}
	if bytes.HasPrefix(prevLabelValuePair, focusLabelEqualBytes) {
		thSeriesCountByFocusLabelValue.push(prevLabelValuePair[len(focusLabelEqualBytes):], seriesCountByLabelValuePair)
	}
	status := &TSDBStatus{
		TotalSeries:                  totalSeries,
		TotalLabelValuePairs:         totalLabelValuePairs,
		SeriesCountByMetricName:      thSeriesCountByMetricName.getSortedResult(),
		SeriesCountByLabelName:       thSeriesCountByLabelName.getSortedResult(),
		SeriesCountByFocusLabelValue: thSeriesCountByFocusLabelValue.getSortedResult(),
		SeriesCountByLabelValuePair:  thSeriesCountByLabelValuePair.getSortedResult(),
		LabelValueCountByLabelName:   thLabelValueCountByLabelName.getSortedResult(),
	}
	return status, nil
}

// TSDBStatus contains TSDB status data for /api/v1/status/tsdb.
//
// See https://prometheus.io/docs/prometheus/latest/querying/api/#tsdb-stats
type TSDBStatus struct {
	TotalSeries                  uint64
	TotalLabelValuePairs         uint64
	SeriesCountByMetricName      []TopHeapEntry
	SeriesCountByLabelName       []TopHeapEntry
	SeriesCountByFocusLabelValue []TopHeapEntry
	SeriesCountByLabelValuePair  []TopHeapEntry
	LabelValueCountByLabelName   []TopHeapEntry
	SeriesQueryStatsByMetricName []MetricNamesStatsRecord
}

func (status *TSDBStatus) hasEntries() bool {
	return len(status.SeriesCountByLabelValuePair) > 0
}

// topHeap maintains a heap of topHeapEntries with the maximum TopHeapEntry.n values.
type topHeap struct {
	topN int
	a    []TopHeapEntry
}

// newTopHeap returns topHeap for topN items.
func newTopHeap(topN int) *topHeap {
	return &topHeap{
		topN: topN,
	}
}

// TopHeapEntry represents an entry from `top heap` used in stats.
type TopHeapEntry struct {
	Name  string
	Count uint64
}

func (th *topHeap) push(name []byte, count uint64) {
	if count == 0 {
		return
	}
	if len(th.a) < th.topN {
		th.a = append(th.a, TopHeapEntry{
			Name:  string(name),
			Count: count,
		})
		heap.Fix(th, len(th.a)-1)
		return
	}
	if count <= th.a[0].Count {
		return
	}
	th.a[0] = TopHeapEntry{
		Name:  string(name),
		Count: count,
	}
	heap.Fix(th, 0)
}

func (th *topHeap) getSortedResult() []TopHeapEntry {
	result := append([]TopHeapEntry{}, th.a...)
	sort.Slice(result, func(i, j int) bool {
		a, b := result[i], result[j]
		if a.Count != b.Count {
			return a.Count > b.Count
		}
		return a.Name < b.Name
	})
	return result
}

// heap.Interface implementation for topHeap.

func (th *topHeap) Len() int {
	return len(th.a)
}

func (th *topHeap) Less(i, j int) bool {
	a := th.a
	return a[i].Count < a[j].Count
}

func (th *topHeap) Swap(i, j int) {
	a := th.a
	a[j], a[i] = a[i], a[j]
}

func (th *topHeap) Push(_ any) {
	panic(fmt.Errorf("BUG: Push shouldn't be called"))
}

func (th *topHeap) Pop() any {
	panic(fmt.Errorf("BUG: Pop shouldn't be called"))
}

func (db *indexDB) DeleteSeries(qt *querytracer.Tracer, tfss []*TagFilters, maxMetrics int) (*uint64set.Set, error) {
	qt = qt.NewChild("delete series: filters=%s, maxMetrics=%d", tfss, maxMetrics)
	defer qt.Done()

	is := db.getIndexSearch(noDeadline)
	defer db.putIndexSearch(is)

	// Unconditionally search global index since a given day in per-day
	// index may not contain the full set of metricIDs that correspond
	// to the tfss.
	metricIDs, err := is.searchMetricIDs(qt, tfss, globalIndexTimeRange, maxMetrics)
	if err != nil {
		return nil, err
	}

	db.saveDeletedMetricIDs(metricIDs)
	return metricIDs, nil
}

// saveDeletedMetricIDs persists the deleted metricIDs to the global index by
// creating a separate `nsPrefixDeletedMetricID` entry for each metricID.
//
// In addition, the deleted metricIDs are added to the deletedMetricIDs cache
// and all the caches that may contain some or all of deleted metricIDs are reset.
func (db *indexDB) saveDeletedMetricIDs(metricIDs *uint64set.Set) {
	if metricIDs.Len() == 0 {
		// Nothing to delete
		return
	}

	// atomically add deleted metricIDs to an inmemory map.
	db.s.updateDeletedMetricIDs(metricIDs)

	// Reset TagFilters -> TSIDS cache, since it may contain deleted TSIDs.
	db.tagFiltersToMetricIDsCache.Reset()

	// Store the metricIDs as deleted.
	// Make this after updating the deletedMetricIDs and resetting caches
	// in order to exclude the possibility of the inconsistent state when the deleted metricIDs
	// remain available in the tsidCache after unclean shutdown.
	// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1347
	items := getIndexItems()
	metricIDs.ForEach(func(part []uint64) bool {
		for _, metricID := range part {
			items.B = append(items.B, nsPrefixDeletedMetricID)
			items.B = encoding.MarshalUint64(items.B, metricID)
			items.Next()
		}
		return true
	})

	db.tb.AddItems(items.Items)
	putIndexItems(items)
}

func (db *indexDB) loadDeletedMetricIDs() (*uint64set.Set, error) {
	is := db.getIndexSearch(noDeadline)
	dmis, err := is.loadDeletedMetricIDs()
	db.putIndexSearch(is)
	if err != nil {
		return nil, err
	}
	return dmis, nil
}

func (is *indexSearch) loadDeletedMetricIDs() (*uint64set.Set, error) {
	dmis := &uint64set.Set{}
	ts := &is.ts
	kb := &is.kb
	kb.B = append(kb.B[:0], nsPrefixDeletedMetricID)
	ts.Seek(kb.B)
	for ts.NextItem() {
		item := ts.Item
		if !bytes.HasPrefix(item, kb.B) {
			break
		}
		item = item[len(kb.B):]
		if len(item) != 8 {
			return nil, fmt.Errorf("unexpected item len; got %d bytes; want %d bytes", len(item), 8)
		}
		metricID := encoding.UnmarshalUint64(item)
		dmis.Add(metricID)
	}
	if err := ts.Error(); err != nil {
		return nil, err
	}
	return dmis, nil
}

// searchMetricIDs returns metricIDs for the given tfss and tr.
func (db *indexDB) searchMetricIDs(qt *querytracer.Tracer, tfss []*TagFilters, tr TimeRange, maxMetrics int, deadline uint64) (*uint64set.Set, error) {
	qt = qt.NewChild("search for matching metricIDs: filters=%s, timeRange=%s", tfss, &tr)
	defer qt.Done()

	if len(tfss) == 0 {
		return nil, nil
	}

	tfKeyBuf := tagFiltersKeyBufPool.Get()
	defer tagFiltersKeyBufPool.Put(tfKeyBuf)

	tfKeyBuf.B = marshalTagFiltersKey(tfKeyBuf.B[:0], tfss, tr)
	metricIDs, ok := db.getMetricIDsFromTagFiltersCache(qt, tfKeyBuf.B)
	if ok {
		// Fast path - metricIDs found in the cache
		if metricIDs.Len() > maxMetrics {
			return nil, errTooManyTimeseries(maxMetrics)
		}
		return metricIDs, nil
	}

	// Slow path - search for metricIDs in the db
	is := db.getIndexSearch(deadline)
	metricIDs, err := is.searchMetricIDs(qt, tfss, tr, maxMetrics)
	db.putIndexSearch(is)
	if err != nil {
		return nil, fmt.Errorf("error when searching for metricIDs: %w", err)
	}

	// Store metricIDs in the cache.
	db.putMetricIDsToTagFiltersCache(qt, metricIDs, tfKeyBuf.B)

	return metricIDs, nil
}

// SearchTSIDs searches the TSIDs that correspond to filters within the given
// time range.
//
// The returned TSIDs are sorted.
//
// The method will fail if the number of found TSIDs exceeds maxMetrics or the
// search has not completed within the specified deadline.
func (db *indexDB) SearchTSIDs(qt *querytracer.Tracer, tfss []*TagFilters, tr TimeRange, maxMetrics int, deadline uint64) ([]TSID, error) {
	qt = qt.NewChild("search TSIDs: filters=%s, timeRange=%s, maxMetrics=%d", tfss, &tr, maxMetrics)
	defer qt.Done()

	metricIDs, err := db.searchMetricIDs(qt, tfss, tr, maxMetrics, deadline)
	if err != nil {
		return nil, err
	}
	if metricIDs.Len() == 0 {
		return nil, nil
	}

	tsids := make([]TSID, metricIDs.Len())
	metricIDsToDelete := &uint64set.Set{}
	i := 0
	paceLimiter := 0
	is := db.getIndexSearch(deadline)
	defer db.putIndexSearch(is)
	metricIDs.ForEach(func(metricIDs []uint64) bool {
		for _, metricID := range metricIDs {
			if paceLimiter&paceLimiterSlowIterationsMask == 0 {
				if err = checkSearchDeadlineAndPace(deadline); err != nil {
					return false
				}
			}
			paceLimiter++

			// Try obtaining TSIDs from MetricID->TSID cache. This is much faster
			// than scanning the mergeset if it contains a lot of metricIDs.
			tsid := &tsids[i]
			err = db.getFromMetricIDCache(tsid, metricID)
			if err == nil {
				// Fast path - the tsid for metricID is found in cache.
				i++
				continue
			}
			if err != io.EOF {
				return false
			}
			err = nil
			if !is.getTSIDByMetricID(tsid, metricID) {
				// Cannot find TSID for the given metricID.
				// This may be the case on incomplete indexDB
				// due to snapshot or due to un-flushed entries.
				// Mark the metricID as deleted, so it is created again when new sample
				// for the given time series is ingested next time.
				if db.s.wasMetricIDMissingBefore(metricID) {
					db.missingTSIDsForMetricID.Add(1)
					metricIDsToDelete.Add(metricID)
				}
				continue
			}
			db.putToMetricIDCache(metricID, tsid)
			i++
		}
		return true
	})
	if err != nil {
		return nil, fmt.Errorf("error when searching for TSIDs by metricIDs: %w", err)
	}

	tsids = tsids[:i]
	qt.Printf("found %d TSIDs for %d metricIDs", len(tsids), metricIDs.Len())

	// Sort the found tsids, since they must be passed to TSID search
	// in the sorted order.
	sort.Slice(tsids, func(i, j int) bool { return tsids[i].Less(&tsids[j]) })
	qt.Printf("sort %d TSIDs", len(tsids))

	if metricIDsToDelete.Len() > 0 {
		db.saveDeletedMetricIDs(metricIDsToDelete)
	}
	return tsids, nil
}

// searchMetricName appends metric name for the given metricID to dst
// and returns the result.
func (db *indexDB) searchMetricName(dst []byte, metricID uint64, noCache bool) ([]byte, bool) {
	is := db.getIndexSearchInternal(noDeadline, noCache)
	defer db.putIndexSearch(is)
	return is.searchMetricName(dst, metricID)
}

func (db *indexDB) SearchMetricNames(qt *querytracer.Tracer, tfss []*TagFilters, tr TimeRange, maxMetrics int, deadline uint64) ([]string, error) {
	qt = qt.NewChild("search metric names: filters=%s, timeRange=%s, maxMetrics=%d", tfss, &tr, maxMetrics)
	defer qt.Done()

	metricIDs, err := db.searchMetricIDs(qt, tfss, tr, maxMetrics, deadline)
	if err != nil {
		return nil, err
	}
	if metricIDs.Len() == 0 {
		return nil, nil
	}

	metricNames := make([]string, 0, metricIDs.Len())
	metricIDsToDelete := &uint64set.Set{}
	var metricName []byte
	var ok bool
	paceLimiter := 0
	is := db.getIndexSearch(deadline)
	defer db.putIndexSearch(is)
	metricIDs.ForEach(func(metricIDs []uint64) bool {
		for _, metricID := range metricIDs {
			if paceLimiter&paceLimiterSlowIterationsMask == 0 {
				if err = checkSearchDeadlineAndPace(deadline); err != nil {
					return false
				}
			}
			paceLimiter++

			metricName, ok = is.searchMetricNameWithCache(metricName[:0], metricID)
			if !ok {
				// Cannot find TSID for the given metricID.
				// This may be the case on incomplete indexDB
				// due to snapshot or due to un-flushed entries.
				// Mark the metricID as deleted, so it is created again when new sample
				// for the given time series is ingested next time.
				if db.s.wasMetricIDMissingBefore(metricID) {
					db.missingMetricNamesForMetricID.Add(1)
					metricIDsToDelete.Add(metricID)
				}
				continue
			}
			metricNames = append(metricNames, string(metricName))
		}
		return true
	})
	if err != nil {
		return nil, err
	}

	if metricIDsToDelete.Len() > 0 {
		db.saveDeletedMetricIDs(metricIDsToDelete)
	}

	qt.Printf("loaded %d metric names", len(metricNames))
	return metricNames, nil
}

var tagFiltersKeyBufPool bytesutil.ByteBufferPool

func (is *indexSearch) getTSIDByMetricName(dst *generationTSID, metricName []byte, date uint64) bool {
	dmis := is.db.s.getDeletedMetricIDs()
	ts := &is.ts
	kb := &is.kb

	if is.db.s.disablePerDayIndex {
		kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixMetricNameToTSID)
	} else {
		kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixDateMetricNameToTSID)
		kb.B = encoding.MarshalUint64(kb.B, date)
	}
	kb.B = append(kb.B, metricName...)
	kb.B = append(kb.B, kvSeparatorChar)
	ts.Seek(kb.B)
	for ts.NextItem() {
		if !bytes.HasPrefix(ts.Item, kb.B) {
			// Nothing found.
			return false
		}
		v := ts.Item[len(kb.B):]
		tail, err := dst.TSID.Unmarshal(v)
		if err != nil {
			logger.Panicf("FATAL: cannot unmarshal TSID: %s", err)
		}
		if len(tail) > 0 {
			logger.Panicf("FATAL: unexpected non-empty tail left after unmarshaling TSID: %X", tail)
		}
		if dmis.Has(dst.TSID.MetricID) {
			// The dst is deleted. Continue searching.
			continue
		}
		dst.generation = is.db.generation
		// Found valid dst.
		return true
	}
	if err := ts.Error(); err != nil {
		logger.Panicf("FATAL: error when searching TSID by metricName; searchPrefix %q: %s", kb.B, err)
	}
	// Nothing found
	return false
}

func (is *indexSearch) searchMetricNameWithCache(dst []byte, metricID uint64) ([]byte, bool) {
	metricName := is.db.s.getMetricNameFromCache(dst, metricID)
	if len(metricName) > len(dst) {
		return metricName, true
	}
	var ok bool
	dst, ok = is.searchMetricName(dst, metricID)
	if ok {
		// There is no need in verifying whether the given metricID is deleted,
		// since the filtering must be performed before calling this func.
		is.db.s.putMetricNameToCache(metricID, dst)
		return dst, true
	}
	return dst, false
}

func (is *indexSearch) searchMetricName(dst []byte, metricID uint64) ([]byte, bool) {
	ts := &is.ts
	kb := &is.kb
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixMetricIDToMetricName)
	kb.B = encoding.MarshalUint64(kb.B, metricID)
	if err := ts.FirstItemWithPrefix(kb.B); err != nil {
		if err == io.EOF {
			return dst, false
		}
		logger.Panicf("FATAL: error when searching metricName by metricID; searchPrefix %q: %s", kb.B, err)
	}
	v := ts.Item[len(kb.B):]
	dst = append(dst, v...)
	return dst, true
}

func (is *indexSearch) containsTimeRange(tr TimeRange) bool {
	if tr == globalIndexTimeRange {
		return true
	}

	db := is.db
	if !db.noRegisterNewSeries.Load() {
		// indexDB could register new time series - it is not safe to cache minMissingTimestamp
		return true
	}
	// The db corresponds to the previous indexDB, which is readonly.
	// So it is safe caching the minimum timestamp, which isn't covered by the db.

	// use common prefix as a key for minMissingTimestamp
	// it's needed to properly track timestamps for cluster version
	// which uses tenant labels for the index search
	kb := &is.kb
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixDateToMetricID)
	key := kb.B

	db.minMissingTimestampByKeyLock.Lock()
	minMissingTimestamp, ok := db.minMissingTimestampByKey[string(key)]
	db.minMissingTimestampByKeyLock.Unlock()

	if ok && tr.MinTimestamp >= minMissingTimestamp {
		return false
	}
	if is.containsTimeRangeSlowForPrefixBuf(kb, tr) {
		return true
	}

	db.minMissingTimestampByKeyLock.Lock()
	minMissingTimestamp, ok = db.minMissingTimestampByKey[string(key)]
	if !ok || tr.MinTimestamp < minMissingTimestamp {
		db.minMissingTimestampByKey[string(key)] = tr.MinTimestamp
	}
	db.minMissingTimestampByKeyLock.Unlock()

	return false
}

func (is *indexSearch) containsTimeRangeSlowForPrefixBuf(prefixBuf *bytesutil.ByteBuffer, tr TimeRange) bool {
	ts := &is.ts

	// Verify whether the tr.MinTimestamp is included into `ts` or is smaller than the minimum date stored in `ts`.
	// Do not check whether tr.MaxTimestamp is included into `ts` or is bigger than the max date stored in `ts` for performance reasons.
	// This means that containsTimeRangeSlow() can return true if `tr` is located below the min date stored in `ts`.
	// This is OK, since this case isn't encountered too much in practice.
	// The main practical case allows skipping searching in prev indexdb (`ts`) when `tr`
	// is located above the max date stored there.
	minDate := uint64(tr.MinTimestamp) / msecPerDay
	prefix := prefixBuf.B
	prefixBuf.B = encoding.MarshalUint64(prefixBuf.B, minDate)
	ts.Seek(prefixBuf.B)
	if !ts.NextItem() {
		if err := ts.Error(); err != nil {
			logger.Panicf("FATAL: error when searching for minDate=%d, prefix %q: %w", minDate, prefixBuf.B, err)
		}
		return false
	}
	if !bytes.HasPrefix(ts.Item, prefix) {
		// minDate exceeds max date from ts.
		return false
	}
	return true
}

func (is *indexSearch) getTSIDByMetricID(dst *TSID, metricID uint64) bool {
	// There is no need in checking for deleted metricIDs here, since they
	// must be checked by the caller.
	ts := &is.ts
	kb := &is.kb
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixMetricIDToTSID)
	kb.B = encoding.MarshalUint64(kb.B, metricID)
	if err := ts.FirstItemWithPrefix(kb.B); err != nil {
		if err == io.EOF {
			return false
		}
		logger.Panicf("FATAL: error when searching TSID by metricID=%d; searchPrefix %q: %s", metricID, kb.B, err)
	}
	v := ts.Item[len(kb.B):]
	tail, err := dst.Unmarshal(v)
	if err != nil {
		logger.Panicf("FATAL: cannot unmarshal the found TSID=%X for metricID=%d: %s", v, metricID, err)
	}
	if len(tail) > 0 {
		logger.Panicf("FATAL: unexpected non-zero tail left after unmarshaling TSID for metricID=%d: %X", metricID, tail)
	}
	return true
}

// updateMetricIDsByMetricNameMatch matches metricName values for the given srcMetricIDs against tfs
// and adds matching metrics to metricIDs.
func (is *indexSearch) updateMetricIDsByMetricNameMatch(qt *querytracer.Tracer, metricIDs, srcMetricIDs *uint64set.Set, tfs []*tagFilter) error {
	qt = qt.NewChild("filter out %d metric ids with filters=%s", srcMetricIDs.Len(), tfs)
	defer qt.Done()

	// sort srcMetricIDs in order to speed up Seek below.
	sortedMetricIDs := srcMetricIDs.AppendTo(nil)
	qt.Printf("sort %d metric ids", len(sortedMetricIDs))

	kb := &is.kb
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs)
	tfs = removeCompositeTagFilters(tfs, kb.B)

	metricName := kbPool.Get()
	defer kbPool.Put(metricName)
	mn := GetMetricName()
	defer PutMetricName(mn)
	for loopsPaceLimiter, metricID := range sortedMetricIDs {
		if loopsPaceLimiter&paceLimiterSlowIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return err
			}
		}
		var ok bool
		metricName.B, ok = is.searchMetricNameWithCache(metricName.B[:0], metricID)
		if !ok {
			// It is likely the metricID->metricName entry didn't propagate to inverted index yet.
			// Skip this metricID for now.
			continue
		}
		if err := mn.Unmarshal(metricName.B); err != nil {
			logger.Panicf("FATAL: cannot unmarshal metricName %q: %s", metricName.B, err)
		}

		// Match the mn against tfs.
		ok, err := matchTagFilters(mn, tfs, &is.kb)
		if err != nil {
			return fmt.Errorf("cannot match MetricName %s against tagFilters: %w", mn, err)
		}
		if !ok {
			continue
		}
		metricIDs.Add(metricID)
	}
	qt.Printf("apply filters %s; resulting metric ids: %d", tfs, metricIDs.Len())
	return nil
}

func removeCompositeTagFilters(tfs []*tagFilter, prefix []byte) []*tagFilter {
	if !hasCompositeTagFilters(tfs, prefix) {
		return tfs
	}
	var tagKey []byte
	var name []byte
	tfsNew := make([]*tagFilter, 0, len(tfs)+1)
	for _, tf := range tfs {
		if !bytes.HasPrefix(tf.prefix, prefix) {
			tfsNew = append(tfsNew, tf)
			continue
		}
		suffix := tf.prefix[len(prefix):]
		var err error
		_, tagKey, err = unmarshalTagValue(tagKey[:0], suffix)
		if err != nil {
			logger.Panicf("BUG: cannot unmarshal tag key from suffix=%q: %s", suffix, err)
		}
		if len(tagKey) == 0 || tagKey[0] != compositeTagKeyPrefix {
			tfsNew = append(tfsNew, tf)
			continue
		}
		tagKey = tagKey[1:]
		nameLen, nSize := encoding.UnmarshalVarUint64(tagKey)
		if nSize <= 0 {
			logger.Panicf("BUG: cannot unmarshal nameLen from tagKey %q", tagKey)
		}
		tagKey = tagKey[nSize:]
		if nameLen == 0 {
			logger.Panicf("BUG: nameLen must be greater than 0")
		}
		if uint64(len(tagKey)) < nameLen {
			logger.Panicf("BUG: expecting at %d bytes for name in tagKey=%q; got %d bytes", nameLen, tagKey, len(tagKey))
		}
		name = append(name[:0], tagKey[:nameLen]...)
		tagKey = tagKey[nameLen:]
		var tfNew tagFilter
		if err := tfNew.Init(prefix, tagKey, tf.value, tf.isNegative, tf.isRegexp); err != nil {
			logger.Panicf("BUG: cannot initialize {%s=%q} filter: %s", tagKey, tf.value, err)
		}
		tfsNew = append(tfsNew, &tfNew)
	}
	if len(name) > 0 {
		var tfNew tagFilter
		if err := tfNew.Init(prefix, nil, name, false, false); err != nil {
			logger.Panicf("BUG: unexpected error when initializing {__name__=%q} filter: %s", name, err)
		}
		tfsNew = append(tfsNew, &tfNew)
	}
	return tfsNew
}

func hasCompositeTagFilters(tfs []*tagFilter, prefix []byte) bool {
	var tagKey []byte
	for _, tf := range tfs {
		if !bytes.HasPrefix(tf.prefix, prefix) {
			continue
		}
		suffix := tf.prefix[len(prefix):]
		var err error
		_, tagKey, err = unmarshalTagValue(tagKey[:0], suffix)
		if err != nil {
			logger.Panicf("BUG: cannot unmarshal tag key from suffix=%q: %s", suffix, err)
		}
		if len(tagKey) > 0 && tagKey[0] == compositeTagKeyPrefix {
			return true
		}
	}
	return false
}

func matchTagFilters(mn *MetricName, tfs []*tagFilter, kb *bytesutil.ByteBuffer) (bool, error) {
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs)
	for i, tf := range tfs {
		if bytes.Equal(tf.key, graphiteReverseTagKey) {
			// Skip artificial tag filter for Graphite-like metric names with dots,
			// since mn doesn't contain the corresponding tag.
			continue
		}
		if len(tf.key) == 0 || string(tf.key) == "__graphite__" {
			// Match against mn.MetricGroup.
			b := marshalTagValue(kb.B, nil)
			b = marshalTagValue(b, mn.MetricGroup)
			kb.B = b[:len(kb.B)]
			ok, err := tf.match(b)
			if err != nil {
				return false, fmt.Errorf("cannot match MetricGroup %q with tagFilter %s: %w", mn.MetricGroup, tf, err)
			}
			if !ok {
				// Move failed tf to start.
				// This should reduce the amount of useless work for the next mn.
				if i > 0 {
					tfs[0], tfs[i] = tfs[i], tfs[0]
				}
				return false, nil
			}
			continue
		}
		// Search for matching tag name.
		tagMatched := false
		tagSeen := false
		for _, tag := range mn.Tags {
			if string(tag.Key) != string(tf.key) {
				continue
			}

			// Found the matching tag name. Match the value.
			tagSeen = true
			b := tag.Marshal(kb.B)
			kb.B = b[:len(kb.B)]
			ok, err := tf.match(b)
			if err != nil {
				return false, fmt.Errorf("cannot match tag %q with tagFilter %s: %w", tag, tf, err)
			}
			if !ok {
				// Move failed tf to start.
				// This should reduce the amount of useless work for the next mn.
				if i > 0 {
					tfs[0], tfs[i] = tfs[i], tfs[0]
				}
				return false, nil
			}
			tagMatched = true
			break
		}
		if !tagSeen && (!tf.isNegative && tf.isEmptyMatch || tf.isNegative && !tf.isEmptyMatch) {
			// tf contains positive empty-match filter for non-existing tag key, i.e.
			// {non_existing_tag_key=~"foobar|"}
			//
			// OR
			//
			// tf contains negative filter for non-exsisting tag key
			// and this filter doesn't match empty string, i.e. {non_existing_tag_key!="foobar"}
			// Such filter matches anything.
			//
			// Note that the filter `{non_existing_tag_key!~"|foobar"}` shouldn't match anything,
			// since it is expected that it matches non-empty `non_existing_tag_key`.
			// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/546 and
			// https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2255 for details.
			continue
		}
		if tagMatched {
			// tf matches mn. Go to the next tf.
			continue
		}
		// Matching tag name wasn't found.
		// Move failed tf to start.
		// This should reduce the amount of useless work for the next mn.
		if i > 0 {
			tfs[0], tfs[i] = tfs[i], tfs[0]
		}
		return false, nil
	}
	return true, nil
}

func isSingleMetricNameFilter(tfss []*TagFilters) bool {
	// We check if tfss contain only single filter which is __name__
	return len(tfss) == 1 && len(tfss[0].tfs) == 1 && getMetricNameFilter(tfss[0]) != nil
}

func (is *indexSearch) searchMetricIDsWithFiltersOnDate(qt *querytracer.Tracer, tfss []*TagFilters, date uint64, maxMetrics int) (*uint64set.Set, error) {
	if len(tfss) == 0 {
		return nil, nil
	}

	var tr TimeRange
	if date == globalIndexDate {
		tr = globalIndexTimeRange
	} else {
		tr = TimeRange{
			MinTimestamp: int64(date) * msecPerDay,
			MaxTimestamp: int64(date+1)*msecPerDay - 1,
		}
	}

	metricIDs, err := is.searchMetricIDsInternal(qt, tfss, tr, maxMetrics)
	if err != nil {
		return nil, err
	}
	return metricIDs, nil
}

// searchMetricIDs returns metricIDs for the given tfss and tr.
//
// The returned metricIDs are sorted.
func (is *indexSearch) searchMetricIDs(qt *querytracer.Tracer, tfss []*TagFilters, tr TimeRange, maxMetrics int) (*uint64set.Set, error) {
	metricIDs, err := is.searchMetricIDsInternal(qt, tfss, tr, maxMetrics)
	if err != nil {
		return nil, err
	}
	if metricIDs.Len() == 0 {
		// Nothing found
		return nil, nil
	}

	// Filter out deleted metricIDs.
	dmis := is.db.s.getDeletedMetricIDs()
	metricIDs.Subtract(dmis)

	return metricIDs, nil
}

func errTooManyTimeseries(maxMetrics int) error {
	return fmt.Errorf("the number of matching timeseries exceeds %d; "+
		"either narrow down the search or increase -search.max* command-line flag values "+
		"(the most likely limit is -search.maxUniqueTimeseries); "+
		"see https://docs.victoriametrics.com/victoriametrics/single-server-victoriametrics/#resource-usage-limits", maxMetrics)
}

func (is *indexSearch) searchMetricIDsInternal(qt *querytracer.Tracer, tfss []*TagFilters, tr TimeRange, maxMetrics int) (*uint64set.Set, error) {
	qt = qt.NewChild("search for metric ids: filters=%s, timeRange=%s, maxMetrics=%d", tfss, &tr, maxMetrics)
	defer qt.Done()

	metricIDs := &uint64set.Set{}

	// Always returns (true, nil) for zero time range used to indicate global
	// index search.
	if !is.containsTimeRange(tr) {
		qt.Printf("indexdb doesn't contain data for the given timeRange=%s", &tr)
		return metricIDs, nil
	}

	if tr.MinTimestamp >= is.db.s.minTimestampForCompositeIndex {
		tfss = convertToCompositeTagFilterss(tfss)
		qt.Printf("composite filters=%s", tfss)
	}

	for _, tfs := range tfss {
		if len(tfs.tfs) == 0 {
			// An empty filters must be equivalent to `{__name__!=""}`
			tfs = NewTagFilters()
			if err := tfs.Add(nil, nil, true, false); err != nil {
				logger.Panicf(`BUG: cannot add {__name__!=""} filter: %s`, err)
			}
		}
		qtChild := qt.NewChild("update metric ids: filters=%s, timeRange=%s", tfs, &tr)
		prevMetricIDsLen := metricIDs.Len()
		err := is.updateMetricIDsForTagFilters(qtChild, metricIDs, tfs, tr, maxMetrics+1)
		qtChild.Donef("updated %d metric ids", metricIDs.Len()-prevMetricIDsLen)
		if err != nil {
			return nil, err
		}
		if metricIDs.Len() > maxMetrics {
			return nil, errTooManyTimeseries(maxMetrics)
		}
	}
	return metricIDs, nil
}

func (is *indexSearch) updateMetricIDsForTagFilters(qt *querytracer.Tracer, metricIDs *uint64set.Set, tfs *TagFilters, tr TimeRange, maxMetrics int) error {
	if tr != globalIndexTimeRange {
		// Fast path - search metricIDs by date range in the per-day inverted
		// index.
		qt.Printf("search metric ids in the per-day index")
		is.db.dateRangeSearchCalls.Add(1)
		minDate, maxDate := tr.DateRange()
		return is.updateMetricIDsForDateRange(qt, metricIDs, tfs, minDate, maxDate, maxMetrics)
	}

	// Slow path - search metricIDs in the global inverted index.
	qt.Printf("search metric ids in the global index")
	is.db.globalSearchCalls.Add(1)
	m, err := is.getMetricIDsForDateAndFilters(qt, globalIndexDate, tfs, maxMetrics)
	if err != nil {
		return err
	}
	metricIDs.UnionMayOwn(m)
	return nil
}

func (is *indexSearch) getMetricIDsForTagFilter(qt *querytracer.Tracer, tf *tagFilter, maxMetrics int, maxLoopsCount int64) (*uint64set.Set, int64, error) {
	if tf.isNegative {
		logger.Panicf("BUG: isNegative must be false")
	}
	metricIDs := &uint64set.Set{}
	if len(tf.orSuffixes) > 0 {
		// Fast path for orSuffixes - seek for rows for each value from orSuffixes.
		loopsCount, err := is.updateMetricIDsForOrSuffixes(tf, metricIDs, maxMetrics, maxLoopsCount)
		qt.Printf("found %d metric ids for filter={%s} using exact search; spent %d loops", metricIDs.Len(), tf, loopsCount)
		if err != nil {
			return nil, loopsCount, fmt.Errorf("error when searching for metricIDs for tagFilter in fast path: %w; tagFilter=%s", err, tf)
		}
		return metricIDs, loopsCount, nil
	}

	// Slow path - scan for all the rows with the given prefix.
	loopsCount, err := is.getMetricIDsForTagFilterSlow(tf, metricIDs.Add, maxLoopsCount)
	qt.Printf("found %d metric ids for filter={%s} using prefix search; spent %d loops", metricIDs.Len(), tf, loopsCount)
	if err != nil {
		return nil, loopsCount, fmt.Errorf("error when searching for metricIDs for tagFilter in slow path: %w; tagFilter=%s", err, tf)
	}
	return metricIDs, loopsCount, nil
}

var errTooManyLoops = fmt.Errorf("too many loops is needed for applying this filter")

func (is *indexSearch) getMetricIDsForTagFilterSlow(tf *tagFilter, f func(metricID uint64), maxLoopsCount int64) (int64, error) {
	if len(tf.orSuffixes) > 0 {
		logger.Panicf("BUG: the getMetricIDsForTagFilterSlow must be called only for empty tf.orSuffixes; got %s", tf.orSuffixes)
	}

	// Scan all the rows with tf.prefix and call f on every tf match.
	ts := &is.ts
	kb := &is.kb
	mp := &is.mp
	var prevMatchingSuffix []byte
	var prevMatch bool
	var loopsCount int64
	loopsPaceLimiter := 0
	prefix := tf.prefix
	ts.Seek(prefix)
	for ts.NextItem() {
		if loopsPaceLimiter&paceLimiterMediumIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return loopsCount, err
			}
		}
		loopsPaceLimiter++
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			return loopsCount, nil
		}
		tail := item[len(prefix):]
		n := bytes.IndexByte(tail, tagSeparatorChar)
		if n < 0 {
			return loopsCount, fmt.Errorf("invalid tag->metricIDs line %q: cannot find tagSeparatorChar=%d", item, tagSeparatorChar)
		}
		suffix := tail[:n+1]
		tail = tail[n+1:]
		if err := mp.InitOnlyTail(item, tail); err != nil {
			return loopsCount, err
		}
		mp.ParseMetricIDs()
		loopsCount += int64(mp.MetricIDsLen())
		if loopsCount > maxLoopsCount {
			return loopsCount, errTooManyLoops
		}
		if prevMatch && string(suffix) == string(prevMatchingSuffix) {
			// Fast path: the same tag value found.
			// There is no need in checking it again with potentially
			// slow tf.matchSuffix, which may call regexp.
			for _, metricID := range mp.MetricIDs {
				f(metricID)
			}
			continue
		}
		// Slow path: need tf.matchSuffix call.
		ok, err := tf.matchSuffix(suffix)
		// Assume that tf.matchSuffix call needs 10x more time than a single metric scan iteration.
		loopsCount += 10 * int64(tf.matchCost)
		if err != nil {
			return loopsCount, fmt.Errorf("error when matching %s against suffix %q: %w", tf, suffix, err)
		}
		if !ok {
			prevMatch = false
			if mp.MetricIDsLen() < maxMetricIDsPerRow/2 {
				// If the current row contains non-full metricIDs list,
				// then it is likely the next row contains the next tag value.
				// So skip seeking for the next tag value, since it will be slower than just ts.NextItem call.
				continue
			}
			// Optimization: skip all the metricIDs for the given tag value
			kb.B = append(kb.B[:0], item[:len(item)-len(tail)]...)
			// The last char in kb.B must be tagSeparatorChar. Just increment it
			// in order to jump to the next tag value.
			if len(kb.B) == 0 || kb.B[len(kb.B)-1] != tagSeparatorChar || tagSeparatorChar >= 0xff {
				return loopsCount, fmt.Errorf("data corruption: the last char in k=%X must be %X", kb.B, tagSeparatorChar)
			}
			kb.B[len(kb.B)-1]++
			ts.Seek(kb.B)
			// Assume that a seek cost is equivalent to 1000 ordinary loops.
			loopsCount += 1000
			continue
		}
		prevMatch = true
		prevMatchingSuffix = append(prevMatchingSuffix[:0], suffix...)
		for _, metricID := range mp.MetricIDs {
			f(metricID)
		}
	}
	if err := ts.Error(); err != nil {
		return loopsCount, fmt.Errorf("error when searching for tag filter prefix %q: %w", prefix, err)
	}
	return loopsCount, nil
}

func (is *indexSearch) updateMetricIDsForOrSuffixes(tf *tagFilter, metricIDs *uint64set.Set, maxMetrics int, maxLoopsCount int64) (int64, error) {
	if tf.isNegative {
		logger.Panicf("BUG: isNegative must be false")
	}
	kb := kbPool.Get()
	defer kbPool.Put(kb)
	var loopsCount int64
	for _, orSuffix := range tf.orSuffixes {
		kb.B = append(kb.B[:0], tf.prefix...)
		kb.B = append(kb.B, orSuffix...)
		kb.B = append(kb.B, tagSeparatorChar)
		lc, err := is.updateMetricIDsForOrSuffix(kb.B, metricIDs, maxMetrics, maxLoopsCount-loopsCount)
		loopsCount += lc
		if err != nil {
			return loopsCount, err
		}
		if metricIDs.Len() >= maxMetrics {
			return loopsCount, nil
		}
	}
	return loopsCount, nil
}

func (is *indexSearch) updateMetricIDsForOrSuffix(prefix []byte, metricIDs *uint64set.Set, maxMetrics int, maxLoopsCount int64) (int64, error) {
	ts := &is.ts
	mp := &is.mp
	var loopsCount int64
	loopsPaceLimiter := 0
	ts.Seek(prefix)
	for metricIDs.Len() < maxMetrics && ts.NextItem() {
		if loopsPaceLimiter&paceLimiterFastIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return loopsCount, err
			}
		}
		loopsPaceLimiter++
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			return loopsCount, nil
		}
		if err := mp.InitOnlyTail(item, item[len(prefix):]); err != nil {
			return loopsCount, err
		}
		loopsCount += int64(mp.MetricIDsLen())
		if loopsCount > maxLoopsCount {
			return loopsCount, errTooManyLoops
		}
		mp.ParseMetricIDs()
		metricIDs.AddMulti(mp.MetricIDs)
	}
	if err := ts.Error(); err != nil {
		return loopsCount, fmt.Errorf("error when searching for tag filter prefix %q: %w", prefix, err)
	}
	return loopsCount, nil
}

func (is *indexSearch) updateMetricIDsForDateRange(qt *querytracer.Tracer, metricIDs *uint64set.Set, tfs *TagFilters, minDate, maxDate uint64, maxMetrics int) error {
	if minDate == maxDate {
		// Fast path - query only a single date.
		m, err := is.getMetricIDsForDateAndFilters(qt, minDate, tfs, maxMetrics)
		if err != nil {
			return err
		}
		metricIDs.UnionMayOwn(m)
		is.db.dateRangeSearchHits.Add(1)
		return nil
	}

	// Slower path - search for metricIDs for each day in parallel.
	qt = qt.NewChild("parallel search for metric ids in per-day index: filters=%s, dayRange=[%d..%d]", tfs, minDate, maxDate)
	defer qt.Done()
	wg := getWaitGroup()
	var errGlobal error
	var mu sync.Mutex // protects metricIDs + errGlobal vars from concurrent access below
	for minDate <= maxDate {
		qtChild := qt.NewChild("parallel thread for date=%s", dateToString(minDate))
		wg.Add(1)
		go func(date uint64) {
			defer func() {
				qtChild.Done()
				wg.Done()
			}()
			isLocal := is.db.getIndexSearch(is.deadline)
			m, err := isLocal.getMetricIDsForDateAndFilters(qtChild, date, tfs, maxMetrics)
			is.db.putIndexSearch(isLocal)
			mu.Lock()
			defer mu.Unlock()
			if errGlobal != nil {
				return
			}
			if err != nil {
				dateStr := time.Unix(int64(date*24*3600), 0)
				errGlobal = fmt.Errorf("cannot search for metricIDs at %s: %w", dateStr, err)
				return
			}
			if metricIDs.Len() < maxMetrics {
				metricIDs.UnionMayOwn(m)
			}
		}(minDate)
		minDate++
	}
	wg.Wait()
	putWaitGroup(wg)
	if errGlobal != nil {
		return errGlobal
	}
	is.db.dateRangeSearchHits.Add(1)
	return nil
}

func (is *indexSearch) getMetricIDsForDateAndFilters(qt *querytracer.Tracer, date uint64, tfs *TagFilters, maxMetrics int) (*uint64set.Set, error) {
	if qt.Enabled() {
		qt = qt.NewChild("search for metric ids on a particular day: filters=%s, date=%s, maxMetrics=%d", tfs, dateToString(date), maxMetrics)
		defer qt.Done()
	}

	// Sort tfs by loopsCount needed for performing each filter.
	// This stats is usually collected from the previous queries.
	// This way we limit the amount of work below by applying fast filters at first.
	type tagFilterWithWeight struct {
		tf               *tagFilter
		loopsCount       int64
		filterLoopsCount int64
	}
	tfws := make([]tagFilterWithWeight, len(tfs.tfs))
	currentTime := fasttime.UnixTimestamp()
	for i := range tfs.tfs {
		tf := &tfs.tfs[i]
		loopsCount, filterLoopsCount, timestamp := is.getLoopsCountAndTimestampForDateFilter(date, tf)
		if currentTime > timestamp+3600 {
			// Update stats once per hour for relatively fast tag filters.
			// There is no need in spending CPU resources on updating stats for heavy tag filters.
			if loopsCount <= 10e6 {
				loopsCount = 0
			}
			if filterLoopsCount <= 10e6 {
				filterLoopsCount = 0
			}
		}
		tfws[i] = tagFilterWithWeight{
			tf:               tf,
			loopsCount:       loopsCount,
			filterLoopsCount: filterLoopsCount,
		}
	}
	sort.Slice(tfws, func(i, j int) bool {
		a, b := &tfws[i], &tfws[j]
		if a.loopsCount != b.loopsCount {
			return a.loopsCount < b.loopsCount
		}
		return a.tf.Less(b.tf)
	})
	getFirstPositiveLoopsCount := func(tfws []tagFilterWithWeight) int64 {
		for i := range tfws {
			if n := tfws[i].loopsCount; n > 0 {
				return n
			}
		}
		return int64Max
	}
	storeLoopsCount := func(tfw *tagFilterWithWeight, loopsCount int64) {
		if loopsCount != tfw.loopsCount {
			tfw.loopsCount = loopsCount
			is.storeLoopsCountForDateFilter(date, tfw.tf, tfw.loopsCount, tfw.filterLoopsCount)
		}
	}

	// Populate metricIDs for the first non-negative filter with the smallest cost.
	qtChild := qt.NewChild("search for the first non-negative filter with the smallest cost")
	var metricIDs *uint64set.Set
	tfwsRemaining := tfws[:0]
	maxDateMetrics := intMax
	if maxMetrics < intMax/50 {
		maxDateMetrics = maxMetrics * 50
	}
	for i, tfw := range tfws {
		tf := tfw.tf
		if tf.isNegative || tf.isEmptyMatch {
			tfwsRemaining = append(tfwsRemaining, tfw)
			continue
		}
		maxLoopsCount := getFirstPositiveLoopsCount(tfws[i+1:])
		m, loopsCount, err := is.getMetricIDsForDateTagFilter(qtChild, tf, date, tfs.commonPrefix, maxDateMetrics, maxLoopsCount)
		if err != nil {
			if errors.Is(err, errTooManyLoops) {
				// The tf took too many loops compared to the next filter. Postpone applying this filter.
				qtChild.Printf("the filter={%s} took more than %d loops; postpone it", tf, maxLoopsCount)
				storeLoopsCount(&tfw, 2*loopsCount)
				tfwsRemaining = append(tfwsRemaining, tfw)
				continue
			}
			// Move failing filter to the end of filter list.
			storeLoopsCount(&tfw, int64Max)
			return nil, err
		}
		if m.Len() >= maxDateMetrics {
			// Too many time series found by a single tag filter. Move the filter to the end of list.
			qtChild.Printf("the filter={%s} matches at least %d series; postpone it", tf, maxDateMetrics)
			storeLoopsCount(&tfw, int64Max-1)
			tfwsRemaining = append(tfwsRemaining, tfw)
			continue
		}
		storeLoopsCount(&tfw, loopsCount)
		metricIDs = m
		tfwsRemaining = append(tfwsRemaining, tfws[i+1:]...)
		qtChild.Printf("the filter={%s} matches less than %d series (actually %d series); use it", tf, maxDateMetrics, metricIDs.Len())
		break
	}
	qtChild.Done()
	tfws = tfwsRemaining

	if metricIDs == nil {
		// All the filters in tfs are negative or match too many time series.
		// Populate all the metricIDs for the given (date),
		// so later they can be filtered out with negative filters.
		qt.Printf("all the filters are negative or match more than %d time series; fall back to searching for all the metric ids", maxDateMetrics)
		m, err := is.getMetricIDsForDate(date, maxDateMetrics)
		if err != nil {
			return nil, fmt.Errorf("cannot obtain all the metricIDs: %w", err)
		}
		if m.Len() >= maxDateMetrics {
			// Too many time series found for the given (date). Fall back to global search.
			return nil, errTooManyTimeseries(maxDateMetrics)
		}
		metricIDs = m
		qt.Printf("found %d metric ids", metricIDs.Len())
	}

	sort.Slice(tfws, func(i, j int) bool {
		a, b := &tfws[i], &tfws[j]
		if a.filterLoopsCount != b.filterLoopsCount {
			return a.filterLoopsCount < b.filterLoopsCount
		}
		return a.tf.Less(b.tf)
	})
	getFirstPositiveFilterLoopsCount := func(tfws []tagFilterWithWeight) int64 {
		for i := range tfws {
			if n := tfws[i].filterLoopsCount; n > 0 {
				return n
			}
		}
		return int64Max
	}
	storeFilterLoopsCount := func(tfw *tagFilterWithWeight, filterLoopsCount int64) {
		if filterLoopsCount != tfw.filterLoopsCount {
			is.storeLoopsCountForDateFilter(date, tfw.tf, tfw.loopsCount, filterLoopsCount)
		}
	}

	// Intersect metricIDs with the rest of filters.
	//
	// Do not run these tag filters in parallel, since this may result in CPU and RAM waste
	// when the initial tag filters significantly reduce the number of found metricIDs,
	// so the remaining filters could be performed via much faster metricName matching instead
	// of slow selecting of matching metricIDs.
	qtChild = qt.NewChild("intersect the remaining %d filters with the found %d metric ids", len(tfws), metricIDs.Len())
	var tfsPostponed []*tagFilter
	for i, tfw := range tfws {
		tf := tfw.tf
		metricIDsLen := metricIDs.Len()
		if metricIDsLen == 0 {
			// There is no need in applying the remaining filters to an empty set.
			break
		}
		if tfw.filterLoopsCount > int64(metricIDsLen)*loopsCountPerMetricNameMatch {
			// It should be faster performing metricName match on the remaining filters
			// instead of scanning big number of entries in the inverted index for these filters.
			for _, tfw := range tfws[i:] {
				tfsPostponed = append(tfsPostponed, tfw.tf)
			}
			break
		}
		maxLoopsCount := getFirstPositiveFilterLoopsCount(tfws[i+1:])
		if maxLoopsCount == int64Max {
			maxLoopsCount = int64(metricIDsLen) * loopsCountPerMetricNameMatch
		}
		m, filterLoopsCount, err := is.getMetricIDsForDateTagFilter(qtChild, tf, date, tfs.commonPrefix, intMax, maxLoopsCount)
		if err != nil {
			if errors.Is(err, errTooManyLoops) {
				// Postpone tf, since it took more loops than the next filter may need.
				qtChild.Printf("postpone filter={%s}, since it took more than %d loops", tf, maxLoopsCount)
				storeFilterLoopsCount(&tfw, 2*filterLoopsCount)
				tfsPostponed = append(tfsPostponed, tf)
				continue
			}
			// Move failing tf to the end of filter list
			storeFilterLoopsCount(&tfw, int64Max)
			return nil, err
		}
		storeFilterLoopsCount(&tfw, filterLoopsCount)
		if tf.isNegative || tf.isEmptyMatch {
			metricIDs.Subtract(m)
			qtChild.Printf("subtract %d metric ids from the found %d metric ids for filter={%s}; resulting metric ids: %d", m.Len(), metricIDsLen, tf, metricIDs.Len())
		} else {
			metricIDs.Intersect(m)
			qtChild.Printf("intersect %d metric ids with the found %d metric ids for filter={%s}; resulting metric ids: %d", m.Len(), metricIDsLen, tf, metricIDs.Len())
		}
	}
	qtChild.Done()
	if metricIDs.Len() == 0 {
		// There is no need in applying tfsPostponed, since the result is empty.
		qt.Printf("found zero metric ids")
		return nil, nil
	}
	if len(tfsPostponed) > 0 {
		// Apply the postponed filters via metricName match.
		qt.Printf("apply postponed filters=%s to %d metrics ids", tfsPostponed, metricIDs.Len())
		var m uint64set.Set
		if err := is.updateMetricIDsByMetricNameMatch(qt, &m, metricIDs, tfsPostponed); err != nil {
			return nil, err
		}
		return &m, nil
	}
	qt.Printf("found %d metric ids", metricIDs.Len())
	return metricIDs, nil
}

const (
	intMax   = int((^uint(0)) >> 1)
	int64Max = int64((1 << 63) - 1)
)

func (db *indexDB) createPerDayIndexes(date uint64, tsid *TSID, mn *MetricName) {
	// Note that even if per-day indexes are disabled (i.e.
	// db.s.disablePerDayIndex == true), we still need to add the entry to this
	// cache because Storage.prefillNextIndexDB() relies on
	// indexDB.hasDateMetricID() to decide whether the index records given
	// metricID need to be created and without this cache the next indexDB
	// prefill will be significantly slower when per-day indexes are disabled.
	db.dateMetricIDCache.Set(date, tsid.MetricID)

	if db.s.disablePerDayIndex {
		return
	}

	ii := getIndexItems()
	defer putIndexItems(ii)

	// Create date -> metricID entry.
	ii.B = marshalCommonPrefix(ii.B, nsPrefixDateToMetricID)
	ii.B = encoding.MarshalUint64(ii.B, date)
	ii.B = encoding.MarshalUint64(ii.B, tsid.MetricID)
	ii.Next()

	// Create metricName -> TSID entry.
	ii.B = marshalCommonPrefix(ii.B, nsPrefixDateMetricNameToTSID)
	ii.B = encoding.MarshalUint64(ii.B, date)
	ii.B = mn.Marshal(ii.B)
	ii.B = append(ii.B, kvSeparatorChar)
	ii.B = tsid.Marshal(ii.B)
	ii.Next()

	// Create per-day tag -> metricID entries for every tag in mn.
	kb := kbPool.Get()
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixDateTagToMetricIDs)
	kb.B = encoding.MarshalUint64(kb.B, date)
	ii.registerTagIndexes(kb.B, mn, tsid.MetricID)
	kbPool.Put(kb)

	db.tb.AddItems(ii.Items)
}

func (ii *indexItems) registerTagIndexes(prefix []byte, mn *MetricName, metricID uint64) {
	// Add MetricGroup -> metricID entry.
	ii.B = append(ii.B, prefix...)
	ii.B = marshalTagValue(ii.B, nil)
	ii.B = marshalTagValue(ii.B, mn.MetricGroup)
	ii.B = encoding.MarshalUint64(ii.B, metricID)
	ii.Next()
	ii.addReverseMetricGroupIfNeeded(prefix, mn, metricID)

	// Add tag -> metricID entries.
	for _, tag := range mn.Tags {
		ii.B = append(ii.B, prefix...)
		ii.B = tag.Marshal(ii.B)
		ii.B = encoding.MarshalUint64(ii.B, metricID)
		ii.Next()
	}

	// Add index entries for composite tags: MetricGroup+tag -> metricID.
	compositeKey := kbPool.Get()
	for _, tag := range mn.Tags {
		compositeKey.B = marshalCompositeTagKey(compositeKey.B[:0], mn.MetricGroup, tag.Key)
		ii.B = append(ii.B, prefix...)
		ii.B = marshalTagValue(ii.B, compositeKey.B)
		ii.B = marshalTagValue(ii.B, tag.Value)
		ii.B = encoding.MarshalUint64(ii.B, metricID)
		ii.Next()
	}
	kbPool.Put(compositeKey)
}

func (ii *indexItems) addReverseMetricGroupIfNeeded(prefix []byte, mn *MetricName, metricID uint64) {
	if bytes.IndexByte(mn.MetricGroup, '.') < 0 {
		// The reverse metric group is needed only for Graphite-like metrics with points.
		return
	}
	// This is most likely a Graphite metric like 'foo.bar.baz'.
	// Store reverse metric name 'zab.rab.oof' in order to speed up search for '*.bar.baz'
	// when the Graphite wildcard has a suffix matching small number of time series.
	ii.B = append(ii.B, prefix...)
	ii.B = marshalTagValue(ii.B, graphiteReverseTagKey)
	revBuf := kbPool.Get()
	revBuf.B = reverseBytes(revBuf.B[:0], mn.MetricGroup)
	ii.B = marshalTagValue(ii.B, revBuf.B)
	kbPool.Put(revBuf)
	ii.B = encoding.MarshalUint64(ii.B, metricID)
	ii.Next()
}

func isArtificialTagKey(key []byte) bool {
	if bytes.Equal(key, graphiteReverseTagKey) {
		return true
	}
	if len(key) > 0 && key[0] == compositeTagKeyPrefix {
		return true
	}
	return false
}

// The tag key for reverse metric name used for speeding up searching
// for Graphite wildcards with suffix matching small number of time series,
// i.e. '*.bar.baz'.
//
// It is expected that the given key isn't be used by users.
var graphiteReverseTagKey = []byte("\xff")

// The prefix for composite tag, which is used for speeding up searching
// for composite filters, which contain `{__name__="<metric_name>"}` filter.
//
// It is expected that the given prefix isn't used by users.
const compositeTagKeyPrefix = '\xfe'

func marshalCompositeTagKey(dst, name, key []byte) []byte {
	dst = append(dst, compositeTagKeyPrefix)
	dst = encoding.MarshalVarUint64(dst, uint64(len(name)))
	dst = append(dst, name...)
	dst = append(dst, key...)
	return dst
}

func unmarshalCompositeTagKey(src []byte) ([]byte, []byte, error) {
	if len(src) == 0 {
		return nil, nil, fmt.Errorf("composite tag key cannot be empty")
	}
	if src[0] != compositeTagKeyPrefix {
		return nil, nil, fmt.Errorf("missing composite tag key prefix in %q", src)
	}
	src = src[1:]
	n, nSize := encoding.UnmarshalVarUint64(src)
	if nSize <= 0 {
		return nil, nil, fmt.Errorf("cannot unmarshal metric name length from composite tag key")
	}
	src = src[nSize:]
	if uint64(len(src)) < n {
		return nil, nil, fmt.Errorf("missing metric name with length %d in composite tag key %q", n, src)
	}
	name := src[:n]
	key := src[n:]
	return name, key, nil
}

func reverseBytes(dst, src []byte) []byte {
	for i := len(src) - 1; i >= 0; i-- {
		dst = append(dst, src[i])
	}
	return dst
}

func (is *indexSearch) hasDateMetricID(date, metricID uint64) bool {
	if is.db.dateMetricIDCache.Has(date, metricID) {
		return true
	}

	var ok bool
	if date == globalIndexDate {
		ok = is.hasMetricID(metricID)
	} else {
		ok = is.hasDateMetricIDSlow(date, metricID)
	}

	if ok {
		is.db.dateMetricIDCache.Set(date, metricID)
	}
	return ok
}

func (is *indexSearch) hasDateMetricIDSlow(date, metricID uint64) bool {
	ts := &is.ts
	kb := &is.kb
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixDateToMetricID)
	kb.B = encoding.MarshalUint64(kb.B, date)
	kb.B = encoding.MarshalUint64(kb.B, metricID)
	err := ts.FirstItemWithPrefix(kb.B)
	if err == nil {
		if string(ts.Item) != string(kb.B) {
			logger.Panicf("FATAL: unexpected entry for (date=%s, metricID=%d); got %q; want %q", dateToString(date), metricID, ts.Item, kb.B)
		}
		// Fast path - the (date, metricID) entry is found in the current indexdb.
		return true
	}
	if err != io.EOF {
		logger.Panicf("FATAL: unexpected error when searching for (date=%s, metricID=%d) entry: %s", dateToString(date), metricID, err)
	}
	return false
}

func (is *indexSearch) hasMetricID(metricID uint64) bool {
	ts := &is.ts
	kb := &is.kb
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixMetricIDToTSID)
	kb.B = encoding.MarshalUint64(kb.B, metricID)
	if err := ts.FirstItemWithPrefix(kb.B); err != nil {
		if err == io.EOF {
			return false
		}
		logger.Panicf("FATAL: error when for metricID=%d; searchPrefix %q: %s", metricID, kb.B, err)
	}
	return true
}

func (is *indexSearch) getMetricIDsForDateTagFilter(qt *querytracer.Tracer, tf *tagFilter, date uint64, commonPrefix []byte,
	maxMetrics int, maxLoopsCount int64) (*uint64set.Set, int64, error) {
	if qt.Enabled() {
		qt = qt.NewChild("get metric ids for filter and date: filter={%s}, date=%s, maxMetrics=%d, maxLoopsCount=%d", tf, dateToString(date), maxMetrics, maxLoopsCount)
		defer qt.Done()
	}

	if !bytes.HasPrefix(tf.prefix, commonPrefix) {
		logger.Panicf("BUG: unexpected tf.prefix %q; must start with commonPrefix %q", tf.prefix, commonPrefix)
	}
	kb := kbPool.Get()
	defer kbPool.Put(kb)
	kb.B = is.marshalCommonPrefixForDate(kb.B[:0], date)
	prefix := kb.B
	kb.B = append(kb.B, tf.prefix[len(commonPrefix):]...)
	tfNew := *tf
	tfNew.isNegative = false // isNegative for the original tf is handled by the caller.
	tfNew.prefix = kb.B
	metricIDs, loopsCount, err := is.getMetricIDsForTagFilter(qt, &tfNew, maxMetrics, maxLoopsCount)
	if err != nil {
		return nil, loopsCount, err
	}
	if tf.isNegative || !tf.isEmptyMatch {
		return metricIDs, loopsCount, nil
	}
	// The tag filter, which matches empty label such as {foo=~"bar|"}
	// Convert it to negative filter, which matches {foo=~".+",foo!~"bar|"}.
	// This fixes https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1601
	// See also https://github.com/VictoriaMetrics/VictoriaMetrics/issues/395
	maxLoopsCount -= loopsCount
	var tfGross tagFilter
	if err := tfGross.Init(prefix, tf.key, []byte(".+"), false, true); err != nil {
		logger.Panicf(`BUG: cannot init tag filter: {%q=~".+"}: %s`, tf.key, err)
	}
	m, lc, err := is.getMetricIDsForTagFilter(qt, &tfGross, maxMetrics, maxLoopsCount)
	loopsCount += lc
	if err != nil {
		return nil, loopsCount, err
	}
	mLen := m.Len()
	m.Subtract(metricIDs)
	qt.Printf("subtract %d metric ids for filter={%s} from %d metric ids for filter={%s}", metricIDs.Len(), &tfNew, mLen, &tfGross)
	qt.Printf("found %d metric ids, spent %d loops", m.Len(), loopsCount)
	return m, loopsCount, nil
}

func (is *indexSearch) getLoopsCountAndTimestampForDateFilter(date uint64, tf *tagFilter) (int64, int64, uint64) {
	is.kb.B = appendDateTagFilterCacheKey(is.kb.B[:0], is.db.name, date, tf)
	kb := kbPool.Get()
	defer kbPool.Put(kb)
	kb.B = is.db.loopsPerDateTagFilterCache.Get(kb.B[:0], is.kb.B)
	if len(kb.B) != 3*8 {
		return 0, 0, 0
	}
	loopsCount := encoding.UnmarshalInt64(kb.B)
	filterLoopsCount := encoding.UnmarshalInt64(kb.B[8:])
	timestamp := encoding.UnmarshalUint64(kb.B[16:])
	return loopsCount, filterLoopsCount, timestamp
}

func (is *indexSearch) storeLoopsCountForDateFilter(date uint64, tf *tagFilter, loopsCount, filterLoopsCount int64) {
	currentTimestamp := fasttime.UnixTimestamp()
	is.kb.B = appendDateTagFilterCacheKey(is.kb.B[:0], is.db.name, date, tf)
	kb := kbPool.Get()
	kb.B = encoding.MarshalInt64(kb.B[:0], loopsCount)
	kb.B = encoding.MarshalInt64(kb.B, filterLoopsCount)
	kb.B = encoding.MarshalUint64(kb.B, currentTimestamp)
	is.db.loopsPerDateTagFilterCache.Set(is.kb.B, kb.B)
	kbPool.Put(kb)
}

func appendDateTagFilterCacheKey(dst []byte, indexDBName string, date uint64, tf *tagFilter) []byte {
	dst = append(dst, indexDBName...)
	dst = encoding.MarshalUint64(dst, date)
	dst = tf.Marshal(dst)
	return dst
}

func (is *indexSearch) getMetricIDsForDate(date uint64, maxMetrics int) (*uint64set.Set, error) {
	// Extract all the metricIDs from (date, __name__=value)->metricIDs entries.
	kb := kbPool.Get()
	defer kbPool.Put(kb)
	kb.B = is.marshalCommonPrefixForDate(kb.B[:0], date)
	kb.B = marshalTagValue(kb.B, nil)
	var metricIDs uint64set.Set
	if err := is.updateMetricIDsForPrefix(kb.B, &metricIDs, maxMetrics); err != nil {
		return nil, err
	}
	return &metricIDs, nil
}

func (is *indexSearch) updateMetricIDsForPrefix(prefix []byte, metricIDs *uint64set.Set, maxMetrics int) error {
	ts := &is.ts
	mp := &is.mp
	loopsPaceLimiter := 0
	ts.Seek(prefix)
	for ts.NextItem() {
		if loopsPaceLimiter&paceLimiterFastIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return err
			}
		}
		loopsPaceLimiter++
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			return nil
		}
		tail := item[len(prefix):]
		n := bytes.IndexByte(tail, tagSeparatorChar)
		if n < 0 {
			return fmt.Errorf("invalid tag->metricIDs line %q: cannot find tagSeparatorChar %d", item, tagSeparatorChar)
		}
		tail = tail[n+1:]
		if err := mp.InitOnlyTail(item, tail); err != nil {
			return err
		}
		mp.ParseMetricIDs()
		metricIDs.AddMulti(mp.MetricIDs)
		if metricIDs.Len() >= maxMetrics {
			return nil
		}
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error when searching for all metricIDs by prefix %q: %w", prefix, err)
	}
	return nil
}

// The estimated number of index scan loops a single loop in updateMetricIDsByMetricNameMatch takes.
const loopsCountPerMetricNameMatch = 150

var kbPool bytesutil.ByteBufferPool

// Returns local unique MetricID.
func generateUniqueMetricID() uint64 {
	// It is expected that metricIDs returned from this function must be dense.
	// If they will be sparse, then this may hurt metric_ids intersection
	// performance with uint64set.Set.
	return nextUniqueMetricID.Add(1)
}

// This number mustn't go backwards on restarts, otherwise metricID
// collisions are possible. So don't change time on the server
// between VictoriaMetrics restarts.
var nextUniqueMetricID = func() *atomicutil.Uint64 {
	var n atomicutil.Uint64
	n.Store(uint64(time.Now().UnixNano()))
	return &n
}()

func marshalCommonPrefix(dst []byte, nsPrefix byte) []byte {
	dst = append(dst, nsPrefix)
	return dst
}

// This function is needed only for minimizing the difference between code for single-node and cluster version.
func (is *indexSearch) marshalCommonPrefix(dst []byte, nsPrefix byte) []byte {
	return marshalCommonPrefix(dst, nsPrefix)
}

func (is *indexSearch) marshalCommonPrefixForDate(dst []byte, date uint64) []byte {
	if date == globalIndexDate {
		// Global index
		return is.marshalCommonPrefix(dst, nsPrefixTagToMetricIDs)
	}
	// Per-day index
	dst = is.marshalCommonPrefix(dst, nsPrefixDateTagToMetricIDs)
	return encoding.MarshalUint64(dst, date)
}

func unmarshalCommonPrefix(src []byte) ([]byte, byte, error) {
	if len(src) < commonPrefixLen {
		return nil, 0, fmt.Errorf("cannot unmarshal common prefix from %d bytes; need at least %d bytes; data=%X", len(src), commonPrefixLen, src)
	}
	prefix := src[0]
	return src[commonPrefixLen:], prefix, nil
}

// 1 byte for prefix
const commonPrefixLen = 1

type tagToMetricIDsRowParser struct {
	// NSPrefix contains the first byte parsed from the row after Init call.
	// This is either nsPrefixTagToMetricIDs or nsPrefixDateTagToMetricIDs.
	NSPrefix byte

	// Date contains parsed date for nsPrefixDateTagToMetricIDs rows after Init call
	Date uint64

	// MetricIDs contains parsed MetricIDs after ParseMetricIDs call
	MetricIDs []uint64

	// metricIDsParsed is set to true after ParseMetricIDs call
	metricIDsParsed bool

	// Tag contains parsed tag after Init call
	Tag Tag

	// tail contains the remaining unparsed metricIDs
	tail []byte
}

func (mp *tagToMetricIDsRowParser) Reset() {
	mp.NSPrefix = 0
	mp.Date = 0
	mp.MetricIDs = mp.MetricIDs[:0]
	mp.metricIDsParsed = false
	mp.Tag.Reset()
	mp.tail = nil
}

// Init initializes mp from b, which should contain encoded tag->metricIDs row.
//
// b cannot be reused until Reset call.
func (mp *tagToMetricIDsRowParser) Init(b []byte, nsPrefixExpected byte) error {
	tail, nsPrefix, err := unmarshalCommonPrefix(b)
	if err != nil {
		return fmt.Errorf("invalid tag->metricIDs row %q: %w", b, err)
	}
	if nsPrefix != nsPrefixExpected {
		return fmt.Errorf("invalid prefix for tag->metricIDs row %q; got %d; want %d", b, nsPrefix, nsPrefixExpected)
	}
	if nsPrefix == nsPrefixDateTagToMetricIDs {
		// unmarshal date.
		if len(tail) < 8 {
			return fmt.Errorf("cannot unmarshal date from (date, tag)->metricIDs row %q from %d bytes; want at least 8 bytes", b, len(tail))
		}
		mp.Date = encoding.UnmarshalUint64(tail)
		tail = tail[8:]
	}
	mp.NSPrefix = nsPrefix
	tail, err = mp.Tag.Unmarshal(tail)
	if err != nil {
		return fmt.Errorf("cannot unmarshal tag from tag->metricIDs row %q: %w", b, err)
	}
	return mp.InitOnlyTail(b, tail)
}

// MarshalPrefix marshals row prefix without tail to dst.
func (mp *tagToMetricIDsRowParser) MarshalPrefix(dst []byte) []byte {
	dst = marshalCommonPrefix(dst, mp.NSPrefix)
	if mp.NSPrefix == nsPrefixDateTagToMetricIDs {
		dst = encoding.MarshalUint64(dst, mp.Date)
	}
	dst = mp.Tag.Marshal(dst)
	return dst
}

// InitOnlyTail initializes mp.tail from tail.
//
// b must contain tag->metricIDs row.
// b cannot be reused until Reset call.
func (mp *tagToMetricIDsRowParser) InitOnlyTail(b, tail []byte) error {
	if len(tail) == 0 {
		return fmt.Errorf("missing metricID in the tag->metricIDs row %q", b)
	}
	if len(tail)%8 != 0 {
		return fmt.Errorf("invalid tail length in the tag->metricIDs row; got %d bytes; must be multiple of 8 bytes", len(tail))
	}
	mp.tail = tail
	mp.metricIDsParsed = false
	return nil
}

// EqualPrefix returns true if prefixes for mp and x are equal.
//
// Prefix contains (tag)
func (mp *tagToMetricIDsRowParser) EqualPrefix(x *tagToMetricIDsRowParser) bool {
	if !mp.Tag.Equal(&x.Tag) {
		return false
	}
	return mp.Date == x.Date && mp.NSPrefix == x.NSPrefix
}

// MetricIDsLen returns the number of MetricIDs in the mp.tail
func (mp *tagToMetricIDsRowParser) MetricIDsLen() int {
	return len(mp.tail) / 8
}

// ParseMetricIDs parses MetricIDs from mp.tail into mp.MetricIDs.
func (mp *tagToMetricIDsRowParser) ParseMetricIDs() {
	if mp.metricIDsParsed {
		return
	}
	tail := mp.tail
	n := len(tail) / 8
	mp.MetricIDs = slicesutil.SetLength(mp.MetricIDs, n)
	metricIDs := mp.MetricIDs
	_ = metricIDs[n-1]
	for i := 0; i < n; i++ {
		if len(tail) < 8 {
			logger.Panicf("BUG: tail cannot be smaller than 8 bytes; got %d bytes; tail=%X", len(tail), tail)
			return
		}
		metricID := encoding.UnmarshalUint64(tail)
		metricIDs[i] = metricID
		tail = tail[8:]
	}
	mp.metricIDsParsed = true
}

// GetMatchingSeriesCount returns the number of series in mp, which match metricIDs from the given filter
// and do not match metricIDs from negativeFilter.
//
// if filter is empty, then all series in mp are taken into account.
func (mp *tagToMetricIDsRowParser) GetMatchingSeriesCount(filter, negativeFilter *uint64set.Set) int {
	if filter == nil && negativeFilter.Len() == 0 {
		return mp.MetricIDsLen()
	}
	mp.ParseMetricIDs()
	n := 0
	for _, metricID := range mp.MetricIDs {
		if filter != nil && !filter.Has(metricID) {
			continue
		}
		if !negativeFilter.Has(metricID) {
			n++
		}
	}
	return n
}

func mergeTagToMetricIDsRows(data []byte, items []mergeset.Item) ([]byte, []mergeset.Item) {
	data, items = mergeTagToMetricIDsRowsInternal(data, items, nsPrefixTagToMetricIDs)
	data, items = mergeTagToMetricIDsRowsInternal(data, items, nsPrefixDateTagToMetricIDs)
	return data, items
}

func mergeTagToMetricIDsRowsInternal(data []byte, items []mergeset.Item, nsPrefix byte) ([]byte, []mergeset.Item) {
	// Perform quick checks whether items contain rows starting from nsPrefix
	// based on the fact that items are sorted.
	if len(items) <= 2 {
		// The first and the last row must remain unchanged.
		return data, items
	}
	firstItem := items[0].Bytes(data)
	if len(firstItem) > 0 && firstItem[0] > nsPrefix {
		return data, items
	}
	lastItem := items[len(items)-1].Bytes(data)
	if len(lastItem) > 0 && lastItem[0] < nsPrefix {
		return data, items
	}

	// items contain at least one row starting from nsPrefix. Merge rows with common tag.
	tmm := getTagToMetricIDsRowsMerger()
	tmm.dataCopy = append(tmm.dataCopy[:0], data...)
	tmm.itemsCopy = append(tmm.itemsCopy[:0], items...)
	mp := &tmm.mp
	mpPrev := &tmm.mpPrev
	dstData := data[:0]
	dstItems := items[:0]
	for i, it := range items {
		item := it.Bytes(data)
		if len(item) == 0 || item[0] != nsPrefix || i == 0 || i == len(items)-1 {
			// Write rows not starting with nsPrefix as-is.
			// Additionally write the first and the last row as-is in order to preserve
			// sort order for adjacent blocks.
			dstData, dstItems = tmm.flushPendingMetricIDs(dstData, dstItems, mpPrev)
			dstData = append(dstData, item...)
			dstItems = append(dstItems, mergeset.Item{
				Start: uint32(len(dstData) - len(item)),
				End:   uint32(len(dstData)),
			})
			continue
		}
		if err := mp.Init(item, nsPrefix); err != nil {
			logger.Panicf("FATAL: cannot parse row starting with nsPrefix %d during merge: %s", nsPrefix, err)
		}
		if mp.MetricIDsLen() >= maxMetricIDsPerRow {
			dstData, dstItems = tmm.flushPendingMetricIDs(dstData, dstItems, mpPrev)
			dstData = append(dstData, item...)
			dstItems = append(dstItems, mergeset.Item{
				Start: uint32(len(dstData) - len(item)),
				End:   uint32(len(dstData)),
			})
			continue
		}
		if !mp.EqualPrefix(mpPrev) {
			dstData, dstItems = tmm.flushPendingMetricIDs(dstData, dstItems, mpPrev)
		}
		mp.ParseMetricIDs()
		tmm.pendingMetricIDs = append(tmm.pendingMetricIDs, mp.MetricIDs...)
		mpPrev, mp = mp, mpPrev
		if len(tmm.pendingMetricIDs) >= maxMetricIDsPerRow {
			dstData, dstItems = tmm.flushPendingMetricIDs(dstData, dstItems, mpPrev)
		}
	}
	if len(tmm.pendingMetricIDs) > 0 {
		logger.Panicf("BUG: tmm.pendingMetricIDs must be empty at this point; got %d items: %d", len(tmm.pendingMetricIDs), tmm.pendingMetricIDs)
	}
	if !checkItemsSorted(dstData, dstItems) {
		// Items could become unsorted if initial items contain duplicate metricIDs:
		//
		//   item1: 1, 1, 5
		//   item2: 1, 4
		//
		// Items could become the following after the merge:
		//
		//   item1: 1, 5
		//   item2: 1, 4
		//
		// i.e. item1 > item2
		//
		// Leave the original items unmerged, so they can be merged next time.
		// This case should be quite rare - if multiple data points are simultaneously inserted
		// into the same new time series from multiple concurrent goroutines.
		indexBlocksWithMetricIDsIncorrectOrder.Add(1)
		dstData = append(dstData[:0], tmm.dataCopy...)
		dstItems = append(dstItems[:0], tmm.itemsCopy...)
		if !checkItemsSorted(dstData, dstItems) {
			logger.Panicf("BUG: the original items weren't sorted; items=%q", dstItems)
		}
	}
	putTagToMetricIDsRowsMerger(tmm)
	indexBlocksWithMetricIDsProcessed.Add(1)
	return dstData, dstItems
}

var (
	indexBlocksWithMetricIDsIncorrectOrder atomicutil.Uint64
	indexBlocksWithMetricIDsProcessed      atomicutil.Uint64
)

func checkItemsSorted(data []byte, items []mergeset.Item) bool {
	if len(items) == 0 {
		return true
	}
	prevItem := items[0].String(data)
	for _, it := range items[1:] {
		currItem := it.String(data)
		if prevItem > currItem {
			return false
		}
		prevItem = currItem
	}
	return true
}

// maxMetricIDsPerRow limits the number of metricIDs in tag->metricIDs row.
//
// This reduces overhead on index and metaindex in lib/mergeset.
const maxMetricIDsPerRow = 64

type uint64Sorter []uint64

func (s uint64Sorter) Len() int { return len(s) }
func (s uint64Sorter) Less(i, j int) bool {
	return s[i] < s[j]
}

func (s uint64Sorter) Swap(i, j int) {
	s[i], s[j] = s[j], s[i]
}

type tagToMetricIDsRowsMerger struct {
	pendingMetricIDs uint64Sorter
	mp               tagToMetricIDsRowParser
	mpPrev           tagToMetricIDsRowParser

	itemsCopy []mergeset.Item
	dataCopy  []byte
}

func (tmm *tagToMetricIDsRowsMerger) Reset() {
	tmm.pendingMetricIDs = tmm.pendingMetricIDs[:0]
	tmm.mp.Reset()
	tmm.mpPrev.Reset()

	tmm.itemsCopy = tmm.itemsCopy[:0]
	tmm.dataCopy = tmm.dataCopy[:0]
}

func (tmm *tagToMetricIDsRowsMerger) flushPendingMetricIDs(dstData []byte, dstItems []mergeset.Item, mp *tagToMetricIDsRowParser) ([]byte, []mergeset.Item) {
	if len(tmm.pendingMetricIDs) == 0 {
		// Nothing to flush
		return dstData, dstItems
	}
	// Use sort.Sort instead of sort.Slice in order to reduce memory allocations.
	sort.Sort(&tmm.pendingMetricIDs)
	tmm.pendingMetricIDs = removeDuplicateMetricIDs(tmm.pendingMetricIDs)

	// Marshal pendingMetricIDs
	dstDataLen := len(dstData)
	dstData = mp.MarshalPrefix(dstData)
	for _, metricID := range tmm.pendingMetricIDs {
		dstData = encoding.MarshalUint64(dstData, metricID)
	}
	dstItems = append(dstItems, mergeset.Item{
		Start: uint32(dstDataLen),
		End:   uint32(len(dstData)),
	})
	tmm.pendingMetricIDs = tmm.pendingMetricIDs[:0]
	return dstData, dstItems
}

func removeDuplicateMetricIDs(sortedMetricIDs []uint64) []uint64 {
	if len(sortedMetricIDs) < 2 {
		return sortedMetricIDs
	}
	prevMetricID := sortedMetricIDs[0]
	hasDuplicates := false
	for _, metricID := range sortedMetricIDs[1:] {
		if prevMetricID == metricID {
			hasDuplicates = true
			break
		}
		prevMetricID = metricID
	}
	if !hasDuplicates {
		return sortedMetricIDs
	}
	dstMetricIDs := sortedMetricIDs[:1]
	prevMetricID = sortedMetricIDs[0]
	for _, metricID := range sortedMetricIDs[1:] {
		if prevMetricID == metricID {
			continue
		}
		dstMetricIDs = append(dstMetricIDs, metricID)
		prevMetricID = metricID
	}
	return dstMetricIDs
}

func getTagToMetricIDsRowsMerger() *tagToMetricIDsRowsMerger {
	v := tmmPool.Get()
	if v == nil {
		return &tagToMetricIDsRowsMerger{}
	}
	return v.(*tagToMetricIDsRowsMerger)
}

func putTagToMetricIDsRowsMerger(tmm *tagToMetricIDsRowsMerger) {
	tmm.Reset()
	tmmPool.Put(tmm)
}

var tmmPool sync.Pool
